From the configuration file format, which is XML but not really, to the fact it doesn't work for Sierra and it won't work for Sierra because the author(s) decided to rewrite the whole app and stopped updating the original one, to the GUI straight out of the 90's... I can't find any other word for describing it. Disaster fits.

Keyboard Maestro is a paid app. It's reasonably cheap, but as a long time Linux user, I was shocked I need to pay at all for something I'd consider an essential part of the OS. But you can try KM for free for some time and decide if it's worth its price. Personally, I'm very happy with it. Its UI could be a bit better, but its functional and lets you easily add and organize your own mappings. Karabiner, on the other hand, is a giant blob of pre-defined mappings you can disable or enable (and adding your own is a pain).

The rewrite of Karabiner promises some of the features of Keyboard Maestro, but it's still underway.