I concur on the power of martial damage builds in BG3. This power rests almost entirely on the -5/+10 feats and on Tavern Brawler*, which BG3 took from "niche feat for grapple specialists" to "the most powerful combat feat in the game." The removal of multiclassing restrictions and the changes to the thief subclass (giving them an extra bonus action every round) then dump several gallons of gasoline on the fire.
So far, the scariest builds I've tried are open hand monk and berserker barbarian, both dipping 3-4 levels into thief and using Tavern Brawler. The berserker throws the Nyrulna spear four times per round, combining massive single-target damage at range and two no-save knockdowns with 8d6 thunder damage in a 20-foot radius (essentially a thunder fireball). The monk spams up to 8 unarmed strikes per round, with a mix of knockdowns, pushes, and stuns as needed. The monk lays down more damage, I think, but the berserker's ability to strike at long range is immensely useful in BG3's complex battlefields.
(Note that this is in Honor Mode; you can get really insane with exponentially scaling damage riders in non-Honor mode, but that's crossing the line from "super-cheesy but legit build" to "exploiting a bug" in my book.)
The scariest caster I've tried was a storm sorcerer dipping two levels in tempest cleric, and abusing the fact that in BG3, dumping water on someone imposes vulnerability to lightning and cold. She could lay down some phenomenal AoE damage, but couldn't sustain it, while the martials could keep pouring on the damage, round after round.
I don't know that this has much if any bearing on 5E, except to say that the -5/+10 feats are OP, which we already knew, and that multiclassing is a mess, which we also knew... and that the 5E designers were wise to keep a tight rein on the action economy. Any time you start handing out extra actions, all hell breaks loose. There's a reason people are willing to dip two whole levels just for Action Surge.
I'm currently taking a break from BG3. When I come back to it, I plan to impose some restrictions on myself (single-class only, no GWM/Sharpshooter/Tavern Brawler), drop back from Honor Mode to Balanced, and focus exclusively on RP. It was fun pushing the limits, but I beat Honor Mode twice and that's enough for this guy. I want to try some of the Origins besides the Dark Urge; I'm thinking of playing Gale and romancing Karlach for a "doomed lovers" theme.
*BG3's version of Tavern Brawler adds your Strength bonus to attack and damage rolls with unarmed strikes and throwing weapons -- and it stacks with your regular Strength bonus. So you can be attacking with +13 to hit, for weapon damage+10, as early as level 6, and that's without magic items and elixirs. Throwing weapon specialists and monks have a field day.
