It used to be a fast program, with a reasonable interface.
For a long time now its interface has been "simplifed" following GNOME 3's User Interface Guidelines, and everything ended up being hidden inside a hamburger menu.
But what definitely made it untenable was not the UX, but its tendency to crash and being really slow under the slightest load.
I was considering contributing to the project, but honestly a better engineered alternative would be welcome.
Thanks for the info (and thanks Kai Willadsen for Meld).