-As mentioned in the sibling comment, stability is a problem.

-Refactoring support isn't great. It's about on par with stock Visual Studio, but stock Visual Studio isn't good compared to VS+R# or IntelliJ. If you're writing "just" an iOS application, I can see this not being a huge deal, but my own use case is bringing over projects for OS X and iOS where I tend to have a lot of code and I'm very used to being able to do massive, contextual refactorings when needed. Can't do that in MonoDevelop.

-It's GTK#, so you have to accept that some stuff just doesn't feel native on a Mac. My mental map of keyboard behaviors no longer properly applies and it's jarring going between it and XCode or IDEA. It's gotten a lot better since I started using it on a Mac, to be sure, but it's not there yet. I compare it here to IDEA, which is also not Mac-native, but manages to feel a lot closer.

Now, one huge, huge plus, that I have to give them a ton of credit for: unlike Visual Studio, MonoDevelop is non-destructive toward project files. This is a power-user beef but a really important one once you get there; if you've hand-edited a csproj, Visual Studio will happily destroy all of your changes (it loads the csproj into an in-memory object model and writes it back out on save). This is infuriating when you have custom stuff in it - something as simple as a wildcard path in a file reference (which is totally fine as per MSBuild rules) will be mashed into a static list of all files at time of invocation. It is super bad, and if MonoDevelop improves some more I might switch to it, even on Windows, solely because of that lack of misfeature.

Not the original commenter, but as someone who's done a lot of work with it: it lacks a lot of the features that I've come to expect from basically any modern IDE, but yet manages to crash twice as much as any other IDE I've ever used (and that includes Xcode).