I would have tended to agree with that statement up to Unity 5.3... since that version, most import woes are gone.
True lots of the problems where fixed but there are still a huge amount of them:
Scale and rotation import problems, a huge pain when doing a lot of models. A lack of collision tools. Doesn't support a back and forth workflow between 3D software and engine; making level design much harder than it needs to be.
Unity doesn't support all animation types, re-targeting animations is a huge pain when compared with Unreal, animation optimizing has a lot of holes. Materials and texture have very limited options unless you use code.Smoothing groups can cause problems with normal maps. Very limited real-time mesh tools. Textures load wrong in webGl when using Chrome or Fox, causing memory leaks.
Last is the limited performance tools and instancing support.
However in Unity's defence, it's better at batching than Unreal, maybe the easiest engine to learn and use. The best engine for single developers, a good market place. Because it doesn't have all these tools it's a light engine with all you need to make a game.
Most of the above problems are only visible in large projects and they do have workarounds or you could use plugins to fix.