It's certainly not worse at authoring smooth UIs, it's just not aaaaalll that much better. While UMG-- UE's 2D toolkit-- has lagged other parts of UE by a lot, it's still way easier to get precise visual results for your UI than in most native UI toolkits, even if it's less mature than, say, CSS. That said, given how elegantly things like 3d collisions are handled in UE, it's pretty ridiculous that you can't have, say, convex hit boxes for buttons in UMG.