My biggest complaint with it is that it looks silly, not that it is notably powerful in any way, nor that it isn't really trivial to "counter"
If the ability to use a shield while TWF does not seem notably powerful in anyway, our view point differs quite a bit; that's fine, just not a lot of common ground we'll find there. My problem is that the narratively silly thing to do is the mechanically powerful thing to do, and it certainly seems unintentional that it works that way. But maybe they read the feedback, are aware it works that way, and thought it was fine! I've been told otherwise by people in this thread that read their articles, but I don't know their minds.
Sure, no one likes broken exploits.... but that requires them to be exploits, broken, or the intended outcome of the rules. I've used some of your content, there are things I've seen in it that I've gone "oof, really?" and looked and found... completely intentional.
So can we separate those things?
I think this is a good illustration of why it doesn't really matter much if it is intentional or not. What matters is the outcome and the quality of the content, and many problems the DM will have so smooth over. If you thought 'oof, really' and found whatever it was was intentionally designed did that make you immediately think it was fine then? Probably not.
It's pretty much exactly my point that WotC should be aiming for at least the same standard as homebrewers or 3rd party content creators (and probably much higher). Imagine the show that would ensue if someone like me posted Giant Insect as my shiny new creation and how people would line up around the goddamn block to explain to me that I didn't understand basic game mechanics.
Either the interactions were intended, and it's terribly balanced, or they were not intended, and its terribly balanced. It's at very least a game warping mechanic that all encounters the DM designs will have to account for and design around in each important fight, which is pretty much my benchmark for broken, at least when it comes to a spell.
If I wrote the sort of things that were busted in D&D 2024, I would be happy to have some bloke like me diligently compiling the mistakes so I could review them... and in fact that exact thing is almost always happening; there's google sheets for just that thing out there. In theory, that's what the UA was, but not only did many things change from the final UA and never get reviewed, they didn't seemingly address the ones that were caught in UA.
I was thinking of Earthbind and Web. And yes, this doesn't have a saving throw... because it is an attack roll. And, again, in 85% of use cases, it won't be used on a creature with legendary resistances. It will be used on non-legendary creatures. And a reduction in speed is only going to shut down enemies that cannot respond at range. Look at the legendary Green Dragon, it may be unable to make melee attacks if you reduce its speed to zero, but it certainly isn't "shut down" in a meaningful way by forcing it to rely on spells and ranged attacks.
So as long as all the legendary creatures have strong ranged options.... is this still a bad thing to rely on their AC instead of legendary resistance? Does the giant insect even have a high attack bonus?
The Giant Insect is a summoned creature, it uses your spell attack roll. So... yes.
You'll find when looking at 5e 2014 spells that no high level spells use a spell attack roll (and for that matter, and D&D 2024 spell besides summons), and spells that do tend only do damage. This is an intentional design, because you're not intended to be able to debilitate a creature through hitting its its AC. Again, that's why Legendary Resistance exist in the first place. Saying it can rely on its AC would be same as 'why can't it rely on its saves?'
As for just accepting the monster cannot move... The only high level 2024 Monster I've seen (the Adult Green Dragon) does not have much it can do to PCs that are >30 feet from it. It's Breath Weapon is 90 feet, and it has a few spells, but outside of that its helpless if it cannot get within 30 feet of it's targets. It's offensive CR is halved by being unable to move and its most dangerous interaction is entirely eliminated, which would make it a fairly trivial fight. A CR 22 creature should have its CR halved by a level 4 spell. That might be a subjective opinion, but it seems pretty reasonable an ask to me.
Its only powerful ranged Legendary Reaction (the new version of Legendary Actions) triggers on using Legendary Resistance... which this would never trigger. It certainly does not seem to me like WotC is designing around the possibility of easily dropping a CR 22's monster's movement speed to 0 to me.
Grapple is a save for PCs making a grapple attempt. I bet some monsters still grapple on a hit, and giant spiders are monsters.
The giant spider summoned by Giant Insect is not the Monster Manual Giant Spider block. It is a custom statblock for the spell (thats how it uses spell attack to hit and has 70 hit points and all that). There is no reason for it to use monster mechanics. It only exists in the context of the spell, and is a PC facing ability.
If anything, the exploit is WEAKER than it used to be, because now moving the enemy around in and out of the spell effect doesn't do anything. You have to move the spell effect, and depending on the exact wording, it might ONLY work if you move the emenation on your turn, dropping us from potentially 12d8 damage before the enemy can react to 3d8.
I don't know where you are getting this. Everything you described still works with the D&D 2024 Spirit Guardians.
It triggers "whenever the Emanation enters a creature's space AND whenever a creature enters the Emanation".