At work, we tried in several different places to us Hamster, all to little effect.
The mail problem is that basically everything in the Ruby ecosystem expects everything to be mutable, and you end up constantly swimming upstream against the mutable currents.
We gave up and said “no more”, and have fallen back to freezing things judiciously where appropriate (mostly class-level constants and the like), and defaulting to not mutating input arguments to functions. Anything that mutates gets a ! at the end of it’s name, so at least you know where your bugs come from ;)