I don't know if someone has lobbied for the shepherd druid yet, but I disagree with the rating. I find that, as a moon druid, my concentration is always best used on a bunch of summons except in very specific situations. Those situations tend to be, more often than not, one of these:
1. There are bad guys with big, scary aoe damage spells that will reliably destroy all of my summons in one shot whether or not they make the save.
2. The bad guys are immune or resistant to mundane animal bites.
The shepherd druid's level 6 ability solves both of those problems. A group of panthers or giant poisonous snakes can, suddenly, actually survive a fireball if they have the hp buff of the shepherd. They can survive it even on a failed save, conceivably, if they also have the bear totem buff.
I don't recall every response I had when I put this out, but basically every rating I made was too high AND too low, depending on whom I was speaking to.
I would first respond that please note that I don't think the Circle of the Shepard archetype is bad, and it doesn't require defending from me, as I haven't attacked it. I gave it a purple rating, which means I think it's fine. The level 6 ability you mention I also rated purple, which means I think it's fine too. It got a purple rating because, like you, I think that solving your issue #2 is worthy use of a archetype ability. I don't think it's amazing
though, and I would be happy to explain why.
First, let's discuss your example. You state that as a Moon Druid, concentrating on summons is very effective. I agree. Here's the thing about Moon Druids though, they are adding their own effective combat form in addition to those summoned creatures, a Shepard Druid is not. Now the Shepard Druid enhances the summons somewhat, which I wouldn't call a great trade-off, but it's OK.
Let's now discuss your 2 scenarios in more detail:
#1: This one is not, in my opinion, as big a deal as you seem to suggest it is. IF you summon a whole bunch of creatures, and IF they are all targeted with a big area of effect spell, I would first suggest that's great! Eating up enemy attacks is the best thing a summoned creature can do for you. Secondly, your suggestiont that they will necessarily die, and that 2 hp per hit die means they will necessarily live isn't exactly true. We both know that right? There are certainly SOME circumstances where those 13 HP constrictor snakes will all die and those 17hp constrictor snakes will all live, but it's not like we made them fireball proof or anything. Frankly, I don't mind my summoned creatures being fragile, I want the enemy to be confident they are fragile, then maybe they attack them.
#2: I agree that this one is helpful. This is why the ability got a purple rating. That said, I would highly disagree it's essential. Let's remember that a Druid is a full caster with all kinds of interesting and effective spells they could be concentrating on. Summoning is only one of those options. If your Druid doesn't have this ability, and you face a Lycanthrope in combat, then maybe moonbeam instead of conjure animals would be a good choice. Spells are a toolkit that provide different tools for different jobs. Providing an ability that allows one tool to be used for more of those jobs is good, but isn't amazing or essential. IMO.
I also think the power of the unicorn totem was not sufficiently credited because I think you have a bias against healing in-combat.
You are not the first to tell me that I have a bias against healing in combat, or that I don't like healing, or I think healing is a waste. All those rumors are myths. It's really exceptionally easy for me to dispel those myths in this case specifically.
I've rated 2 druids archetypes in total in this guide. You are criticizing my choice of which of the two I ranked lower, but what is the primary ability of the one I ranked higher? A healing ability. Furthermore, that healing ability is the highest ranked ability for either archetype. If your accusation were true, shouldn't we expect me to rank it poorly? I think that makes it pretty obvious I think in combat healing is good, valuable, and just like you, I think it's better when it uses less of our action economy.
Here's why these myths persist even though they are so clearly untrue. It's because in my Wizard guides I have routinely suggested that "Healer" as a primary combat role is terrible. D&D isn't WOW, and a dedicated combat healer is not only not essential, it's a waste. You simply can't heal enough to make healing worth your actions in combat. Remember, in D&D if you go down, you don't die, and one HP of healing brings you back up. That's one HP as in 1/10 of a single goodberry spell. If that goodberry is delivered by the Wizard's familiar, then it doesn't even use a bonus action! That's some great in-combat healing.
Effective in-combat healing therefore should be something that doesn't require all your attention and abilities. I therefore consider healing that can be used with little action economy to be superior. A Circle of Dreams druid can heal as a bonus action at up to 120' range, for a significant quantity, and provide temp HP on top of that, and still cast a non-cantrip spell with their main action, and do the same again next round. That's solid in-combat healing and healing in-combat I think is completely worthwhile, valuable and effective.
But! The unicorn totem offers the -best- kind of in-combat healing! A small, cheap, widely applied bonus action heal!
To be clear, the unicorn totem does not offer any healing at all. What it does is enhances other healing spells you cast. This means in addition to setting up this ability, you need to prepare and cast healing spells.
With that up, if you healing word on subsequent turns you're throwing out a mass healing word, but without the much higher cost. That's not going to do much if you use healing to combat incoming damage, but it will get multiple people up at once for a very low resource cost, and it leaves your action unused.
Yes, you essentially turn healing word into a mass healing word. Not for free mind you, you need to use your primary achetype ability to set this up.
More importantly, when you say "and it leaves your action unused", let's not forget that although your action is unused, it does become restricted, as in, no casting a spell except for a cantrip. Also keep in mind, you aren't a Moon Druid, so you aren't using that action to maul things with your bear claws. Now maybe you have a Moonbeam up and you are using that action to move it, which is reasonable, but you aren't doing that if you are concentrating on summoned creatures. If you are, that action is probably a damage-cantrip, which is fine, but not really that valuable.
I'll also pick on your use of the words, "on subsequent turns", because, yes, this is a TWO round set up. As in, this isn't a good healing emergency response.
Let's just compare to the Circle of Dreams Druid's healing ability, that requires no set up, has superior range, can be used on the same round you cast a non-cantrip spell, and buffs with additional temp HP. Except for the "mass" portion its objectively superior(which isn't always all that valuable. I've been in lots of combats where one party member takes the brunt of the enemy attack. In fact, that might even be the norm).
That said, using the unicorn totem as an enabler for mass healing with first level slots isn't bad, and I don't mean to suggest it is, only to suggest that it's a decent option for the Druid to use, but not an amazing one. The totem ability in general is pretty decent, it gives multiple options that are all not bad. Thus the purple rating.
Hopefully that explains my reasoning for my "It's a decent choice" ranking for this archetype. I would repeat, as I say in the guide, that if Summoning is what you want to focus on, Circle of the Shepard Druid is the best option out there. Really, if you want to focus on summoning, that magic-attacks ability alone is probably essential. I don't think however that a summoning focus is essential for Druids in general. It's one decent option in a bunch of decent options.