Sacred Flame is based on Dexterity, not Wisdom (it says so in the tooltip). If you don't have a lot of Dexterity then Sacred Flame is only useful against sleeping/unconscious/prone targets.
no. its a confusing/terrible tooltip description in game but it says "attack/save dexterity" because it attacks their dex stat and they have to make a dex save. Your wisdom increases the DC of the check as normal per 5e rules.
as for the topic, I agree the rng seems kinda wonky/weighted, but its likely just bad luck. the probabilities you guys are talking about are misleading; while its true getting a certain number a bunch of times in a row is an infinitesimally small chance, the probability of rolling a certain number is the same no matter how many times you roll it. if you flip a coin, you have a 50/50 chance of it being heads. the next time you flip it, still 50/50. and so on.
as far as the enemies, some of them have definitely been tweaked. the most egregious one I've found is the Minotaurs. 5e minotaurs don't have a long range leap that does AoE damage and knockdown, nor do they have 3 multiattacks. They have a charge/rush, a gore attack, and a greataxe attack. multiattacks are broken currently it seems and will often not use up an action allowing even more attacks