By the way I was curious if this topic had been discussed on the ERPNext forum and indeed it was:
Some users there have some valid complains wrt Tryton (but @dave is working on the point that is for me one of the biggest : our documentation … thanks a lot!). The UI seems both a blessing and a curse so I would not consider it a complain 
For the biggest issue I had with ERPNext / Frappe was the duplication of the business logic in Python and JS. From my POV this is a big warning sign. But I don’t know much more about it (apart from the fact that indeed their UI is more modern).