5 levels in ranger isn't really specific to gloom stalker imo, I'd go 12 gloomstalker over 12 hunter, and I honestly would just never pick beast master at all.
For rogue, the house rule extra bonus action per round is frankly broken and they should have done something else instead. Actions are the most valuable combat resource, and anything that breaks the action economy (eg thief, how they implemented haste) is always going to stupid overpowered. Other than that single spec rogue gets the sneak attack progression because they don't get extra attacks, and if you go dual wield you get 3 attacks per round as a thief anyway which is the same as eg a dual wield ranger.
I'd consider the warlock extra attack stacking a bug, it explicitly doesn't work like that in pen & paper. I also wouldn't consider warlock OP since there's not really anything stopping you long resting after every combat so other casters can use all their best spells every combat.
The wizard subclasses are mostly weak, but that's really just because the base wizard class is strong enough that the subclasses don't really matter. Same thing with cleric domains really, they're meant to be the icing on the top of a already very solid class not the main feature.
Personally I find throwing a flask of water at someone so your lightning spell does double damage as stupid as putting down 5 barrels next to someone and vaporizing them, but Larian things. That's all house rule stuff.
The saving throw replaces the hit chance so ideally both sacred flame vs ray of frost would both average ~70%ish hit rate, with the more accurate depending on creatures saves. Unfortunately there's a lot of stuff in this game that boosts attack rolls and not a lot of enemies with really bad dex saves, so the attack roll spells just end up being better virtually all of the time.
Bard is a good skill monkey but you don't really need lockpicking and stealth on your conversation person so calling it the best class is a bit of a stretch imo. You can do the same thing with warlock or sorcerer anyway, both those classes benefit a lot from dex.
Overall I would point out that while 5th edition d&d definitely has it's faults, a lot of what I'd consider to be issues in this game stem from the house rules and custom item design that introduce a whole lot more balance issues.