If it's early in the life of a project and you don't have much experience with the framework, tiny annoyances are a warning sign. If the easy things don't work the way I like, what happens when I try to do something hard? The cost of switching is low.

I used merb for all of about 10 minutes before giving up on it. I fooled around, created a couple very basic crud pages, and it worked for a bit, then I got nothing but error pages. root cause: I had created a new class called 'fish'. That opened my eyes to the fact that were was so much automagic fairy dust sprinkled about, if anything serious ever went wrong I would never figure it out.