I use WIndows. MameUI is a windows app. That brings along all sorts of things the dev's will know about - being able to resize the window, tabs, minimize and maximize button, dialog boxes, drop down lists, scroll bars.
I'm not saying the new UI for Mame is horrible - but it is a huge step backward in usability. Most of what it is missing is related to the normal standards that Windows apps demand. I see the new built-in UI as a backup, something to use if forced too. It' not user-friendly. It's 2016, not 1990.
As for features, Image Enhancements, as I've stated elsewhere, is a deal breaker for me. Since the built in UI doesn't have them, I won't use it. I'd rather simply not upgrade that use the new built-in UI the whole time.
I like being able to easily change the view by adding and taking away panels (for example, I don't always need the Folder List, and it's easy to show it, and then take it away).
In fact, being able to customize the fonts in the UI, the background image, the color of the font etc.... it all is better in MameUI.
That's right now, I'll add if there's anything else.
For the record, MameUI isn't perfect by any means, particularly when it comes to usability - but it's a long way ahead of the new built-in UI.