It's not the complexity. It's the bloat, terrible interactions and game dynamics. 3.5 didn't suffer from gm burn out, but 5e does. Because 5e is a bigger mess.
Not on 3.5 per se, but I had years long GM burnout after running my first Pathfinder 1e campaign. Bad memories from it were what actually kept me from giving Pathfinder 2e a chance for a long time.