I love familiars. I am playing a wizard right now who uses an owl (named Who).
But they're not optimal for scouting. For these reasons:
1. They die often. One swat and they're toast. One area effect and they're toast. So many things kill them.
And then you need time to re-summon them as a ritual. Which takes 1 hour and 10 minutes. In the dungeon we're in right now (White Plume Mountain) that's 11 wandering monster checks. And because you're casting a spell that whole time, you don't even get all the benefits of a short rest for it.
2. They're not nearly as good at stealth as the rogue. +3 and tiny size isn't bad, but it's no rogue. So they can get caught. And then...see #1.
3. They cannot open doors, or even lift things like rugs to look under it. That 3 strength and Tiny size and lack of a hand/fingers just doesn't cut it to do much. When a simple door stops you from scouting further, that can be an issue.
4. They have a range of 100 feet. Which is good, but sometimes it's nice to be able to go further than that.
5. You're deaf and blind while seeing through your familiar. Which means you're basically auto-surprised if foes come on your resting spot while you're telepathically looking through the eyes of your familiar.
I'd take a rogue over an owl to scout any day. Familiars are fantastic. But they're a poor-mans rogue for scouting. The real deal is better.
I definitely agree with a lot of this, but it's one-sided, because you focus solely on where they're worse.
Re: the individual points.
1. True - however, if your familiar is out in combat, unless the situation is kind of desperate or you don't really need them, you're doing it wrong. And they rarely get killed in scouting, and in any situation where they do, that's probably a situation where a Rogue would have taken a lot of HP of damage and even possibly been killed themselves.
2. Unarguable. Rogues are better at Stealth. It's not very plausible that a 6'4" 220lb Tiefling covered armour and weapons is, but he is. However, re: "see no.1", I would point out that unless you are fighting a particularly unusual foe, or your familiar is hilariously out-of-place, that's not likely to be the result of being discovered. It's less likely someone is going to murder a mouse than an owl, say.
3. Definitely true. Mice or the like can get under/through virtually any non-magical door, realistically, but they aren't owls, which are generally otherwise the most optimal kind of familiar.
4. Yes though this highlights an issue with Rogues too, that being that they have no way to communicate with the party, and you can let the familiar go further if you are willing to deal with communicating with it.
5. Not an issue unless you're solo and unprotected for some bizarre reason.
I think what you quite correctly demonstrate is for ultra-high-intensity dungeons (wandering monster checks every 10 minutes whilst stationary is berserk 1E-style stuff, in fact most 1E adventures had lower check rates as I recall), and probably any fairly intense dungeon, Rogue is your better option.
However that shows the problem. You're comparing a played character class, with an actual player behind it and so on, to a single spell from other characters.
Advantages familiars have include:
1) Even if spotted, many familiars will not cause enemies to react. There will be high-security situations where they will, but you can't just murder every raven, mouse or owl you see (I admit in a "semi-realistic" or "lived in" fantasy setting, owls might well be seen with suspicion
). Rogues are much less likely to be able to get away with this, though it's not unheard-of.
2) Many familiars can fly. This is huge outside dungeons. It's much more useful even than a Winged Tiefling Rogue or the like, even, because again, if people see a bird, they think nothing of it, even if penetrates their consciousness, whereas a some galumphing Tiefling lumbering through the skies will attract considerable attention (I play a Winged Tiefling Bard in one game, and I always try and get above where I'd like to fly to, if I can so I can glide in and hopefully be more stealthy as a result).
3) Familiars die easily, but if they die, the consequences are low. Rogues can penetrate further and do more, but if they get caught, they're somewhat likely to take considerable damage or even get killed (or captured) before they get away. All it takes is one good grapple from some Hobgoblin or whatever and it's basically all over. They do have good tools to escape at least, in 5E.
4) Familiars can rely what they see directly to the caster (within range). Rogues on the other hand, have to get back to the party, which can often be a whole other set of checks and so on. This is often more a meta-problem, because it can turn the game into "The Rogue's Big Adventure" for minutes or more, slow the game down, and bore people. I found actually giving our the Rogue in the group I run the equivalent of text-messaging (a stone tablet linked to another stone tablet, if he drew on it in chalk, the writing also appeared on the other, and vice-versa) actually made the game feel better and run faster, though I can see why such Earthdawn-y stuff isn't default part of D&D.
Really it's a lot like sending a small, cheap drone in a cyberpunk game like SR or CP2020 (more the latter, SR drones tend to be ludicrously overpriced and you can get feedback when they get destroyed, or could in the last edition I played), vs sending an actual person to stealth in. They both have their role, and both have limitations when substituting for the other.
The best possible scenario, in my experience is when you have two-three party members with excellent Stealth who can move together as a team, and who have a familiar with them (or better yet, the entire party, but that's rare - though Pass Without Trace will help a lot).
