1. Customizing views requires that you use Arbre. Learning an entirely new tool for generating webpages when every developer is already familiar with partials, html, erb, etc seemed backwards to me.
2. I tried to design upmin-admin so it was easier to setup. Active admin requires a little bit more work (but truthfully not a ton more).
3. Adding things like actions was significantly easier in upmin-admin. You basically just say what method you want to have on an admin page and it makes it work.
That said, this is very early stage (beta at best) and active admin has been under development for a while, so active admin is definitely more polished right now. That should change over time.