Backstabbing is really a variation of optional flanking rule, however I do agree that ditching out the flanking requirement (which basically is ganging up on a single enemy) makes advantage a bit too easy to come by.
On the other hand it's weird that I've never seen anyone complaining about threatened rule, it should only be applied to ranged attack when you are in melee range. Meanwhile in BG3 it seems to be a small aura.

About the Armor&HP changes, really ? Sure shatter no longer instantly kills a bunch of goblins, but what you are seeing is actually a modest nerf to the monster. Goblins stayed at CR 1/8 bracket HP wise (7-35), while their armorclass crashed pretty hard (nerf by just 2 AC is lowering them by bracket, their CR armor wise dropped to 0) (DMG p274), but in the end we take the average which puts them at CR 1/8, instead of 1/4 as in MM. As far as homebrewing goes it's pretty much sticking to the D&D rulebooks. And even if we ignore the DMG, majority of your offensive casts will be (or at least should be) cantrips, majority of which are attack rolls which are benefiting from the change.