It comes down to a few things. One, it messes with bounded accuracy by allowing you to boost your AC by a huge amount for 1 round. That's not a big deal at low levels, but it doesn't take long before you have a lot of level 1 slots and you're now doing much of substance with them, so there's really no cost to loading up on Shields.
Two, it basically can take away a monster's turn as a reaction. It's generally only used when you would take a hit, and then you not only turn that into a miss retroactively, the +5 AC has a good chance to turn off their other attacks as well. Most enemies want to target your AC, and this means that Shield can be better than Counterspell, especially since it protects against OTHER enemies attacking you.
And then there's just the general hatred some DM's have for being told "um, actually", lol. Messing with their die rolls, forcing rerolls, or simply saying "nuh uh, you miss!" just rubs some people the wrong way (go find a Silvery Barbs debate if you want proof).
And finally, it's easy to get the ability to use Shield even if you're not playing a class that gets it natively.
I'm not defending any of these stances; personally, I have yet to use Shield because I want more mileage out of a spell slot than a one-turn safety net, and I don't have an issue with it, but these are complaints people have. I'm also not going to say my point of view is "correct"; it just doesn't suit my playstyle.
I'd love to go back to Shield just being an AC buff for the combat instead of this weird "counter attacks and oh magic missile too" spell.
You can push the edges of bounded accuracy but you need to build for that if you want to have lots of slots to do it with and there are tradeoffs for that.
I agree with a lot of the rest of that, but don't see it as much of a problem, especially when enemy casters have it too.
One thing I disagree with is that it is always easy to get shield. That depends on the class. It is super easy for a Bard, Paladin or Artificer and pretty easy for a Warlock (although they will not have a lot of slots). It is more difficult to get on more than a once a day cast for most other classes that don't have it on their list. You could design for it with a full or half caster, but you need to either pick specific subclasses or maintain a high intelligence or Charisma. Not terribly difficult, but not automatic either.
I have played other versions of the game and I like the current shield a lot better.