FWIW, my experience on projects with a rails backend is it makes it really easy to fall into performance traps. Active Record is especially bad at encouraging N+1 queries, though the same could be true of many ORMs.
A second issue is that if you come from a background where everything is either explicit or obvious, you are in for a world of hurt. I've seen senior developers try to figure out where a particular method is defined by setting a breakpoint, connecting a REPL, then invoking some special methods to get Ruby to tell them the file and line for the source definition.
It is just a different way of developing, and (personally) I think the reputation is a bit cult-ish.