I'm working with Maxwell since V1. In Sketchup, 3DSmax and Studio - Studio mostly for preparing scenes for final rendering (pack&go, etc.) and material testing.
I'm not sure if i understand your question. Are you speaking about the separate Maxwell for Sketchup or the plugin?
I would say the general problem with SU is the limited polycount, but you can use mxs references for complex objects - of course you have to prepare them in an high poly capable external app (like max). And then there are some features that are missing in the SU-plugin like the ability to apply multiple gras modifiers to an object - i don't know if this has changed in V4, since i didn't try it... i'm still on V3.
I have to say that i really don't like the way the development of Maxwell has taken in the last years and especially with V4. There are many features that are still missing regarding archviz, it looks like research and development is almost non-existent at Next Limit or most of the resources have been concentrated on a the very limited GPU engine and the new licensing system with ridiculous overpriced updates...
I'm really finally considering to jump the ship and switch to v-ray (i already have a license for v-ray 3 for Max) or corona - after using Maxwell for more than 11 years. Their behaviour is really unbelievable...
(and i'm considering to switch to max for modeling because of the "development" of SU)