On the contrary. Compilers written in a high-level language are at a distinct advantage: they're easier to optimize (significantly so), easier to debug, and support forms that are difficult to deal with in a low(er)-level language like C or C++.

I do compiler development every day, and I no longer touch low-level compilers. Everything I write is Boo (for my OS), Ruby (for my startup -- by far my favorite language for compiler dev, despite not liking it in general), or Python. As a result, I have far more maintainable code than the majority, and the code I output is incredibly well optimized.

Edit: To clarify, by 'support forms that are difficult to deal with ...' I mean things like using a pure S-exp structure. Rather than a traditional intermediary form, you can represent your compiler state as an S-exp and iteratively optimize and compile it. Standardizing around a form like that, when it's easy to deal with, greatly simplifies code. (This is actually the reason Ruby is my compiler language of choice these days.)