To OP and most of the other posters : GWM and SS are NOT the uber feats because CONTEXT and you deal no damage if you MISS.
Duelist is a strong thing, especially if combined with a shield. Is it overpowered? No, many things are strong in 5e, sneak attacks, smiting, sorcs and warlocks going nova, cleric with spirit guardians up etc etc.
You can doe loads of heavy hitting builds in 5e without to much system mastery.
There ill always be one monster which gets you into trouble:
e.g
- high AC cancels SS and GWM
- high HP cancels out severe base damage aka duelist
- casting cancels out melee
- very heavy hitting mobs can nullify casters and low AC squishies
- flying mobs vs melee experts
- invisible mobs sneaking up on archers and other ranged etc etc etc.
I hope you see the point.