You will indeed have to re-implement stuff Rails gives you. But more importantly, you will have to maintain it.

I maintain a large-ish Sinatra app that should've been done with Rails. Update cycles on the gems are a nightmare. It feels cobbled together. it's a "half baked homegrown version of Rails."

Someday I will just move the logic to a new Rails app and be done with it. It was a terrible idea.

* your experience may vary. :)