Usually, the problem with this is designing armour that fits a shifting model.
MMO's often get away with this by having low quality armour design, where most things are based on a skin texture rather than actually modelled items (Basically, everything is a skintight suit...)
When games model individual items, it's harder to use body morphing because the items remain static in size while you morph your body. Thus they have to create multiple copies of the items to account for this (This is true for even games where there is no body morphing and it's just male/female options. 2 copies of every single item tuned to fit each gender's body sizes). Unless they simply try to morph it and let it look ugly with bunches of clipping and texture stretching.
For Larian, things get even more complicated. As they are utilizing motion capture to do animations. Which only really works when all the bodies will use the exact same structure that the motion capture model has. Any morphs away from that will cause the animations to not work as the bones are in the wrong place when being manipulated. You can see similar things happen in older games, where they might not even be using mocap but they clearly animated around a base model for the face, so more extreme morphs look awful (I recall this happening to my characters in DA:I a lot. Trying to make a not-ugly face can lead to really uncanny animations...)
As far as plate armour goes...
Joints are generally fine to be flexible. Given that they are specifically made out of overlapping plates that slide underneath each other to allow for movement (With of course, places like armpits and back of the knee being exposed, usually with mail covering them, later they also incorporated dangling armpit protectors to further shore up that particular vulnerability)
As for things like the breastplate acting as though it's made of rubber... Yeah, that's not realistic. Though it comes down to the animation problem. All the animations of a character were done as if the character was unencumbered by any armour and they didn't bother to make any with a character more restricted by a solid piece of plate, so you get these rubbery "Plates", this is also often exacerbated for boots because games love to make boots entirely out of metal rather than actual armour which is just plates that go over the top of someone's boots (Or in a lot of modern cases, trainers)
Of course, a way to alleviate this is by using overlapping plates to allow for movement, not very realistic (Since it provides a very big vulnerability by having that gap between plates vulnerable to a weapon being thrust in, both directly and also the weapon can simply glance and then catch on the next plate and be forced into the gap. Not only that, but the plates would have to be overall thinner to allow this to work) but at least better than solid steel acting like rubber.
Or you know... Get some realistic armour for your mocap models to wear for some additional animations... Just make sure it's actually fitted for them and not some pre-made junk (As oversized armour will greatly affect mobility when wearing it)