Thing is, protection on a pushbike is quite hard. It’s good for protecting against scrapes, bruises, hard edges etc but a lot of the injuries we get are basically our own bodies hurting ourselves with momentum when we stop too fast and that’s very hard to prevent, without a lot of bulk. Ribs especially are mostly about this, same with shoulders, collarbones. It’s not that the protective shirts etc are useless, they’re definitely not but they’re quite specific, so while you protect against a bunch of injuries they make very little difference to others. So it’s fairly unlike a proper kneepad, for example, which is effective pretty much all the time.
(also, some mtb suits are very influenced by motocross gear, and that’s a whole different use case, we just don’t benefit much at all from chest roost guards in most crashes but mtb kit keeps on getting them anyway, almost always at the expense of some more useful protection. Still, it might just save your life if you ride into a spiky branch)
I always think of it as being a bit like cars- crumple zones and bumpers don’t help much when you’re hit by all the heavy stuff you had in the back seat.
TLDR modern suits can be pretty comfy to ride in, sweatier but not as bad as you’d think. Just don’t expect it to always work.