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.
Haven’t you read the whole post? My main complaint is how this lower ac undermine the later spells efficiency. I think the armor class is more of a threat than the hp issue.
Undermine the efficiency ? Only of the spells, and abilities that force a saving throw. Spells that require an attack roll are buffed by this change, which are the majority of your casts anyway. But even then it hardly matters, because Shatter dropping a bunch of goblins to two HP means they will be very likely moped down in the same turn. 8 AC is pretty much 90% to hit chance, as opposed to 50% chance at AC 15, and it's very likely that the party damage output at very least stays the same (because the fighter or rogue who were going to miss their attacks will finish off the goblins), meaning you've still spent your premium spellslot pretty well. And that's assuming you want to commit to casting Shatter, instead of throwing down cantrips at chaff.