a) Eclipse itself is less usable and less stable. I have colleagues who are unable to debug, experience crashes etc. which I never see on Linux / Windows.
b) probably you are doing java development. Java itself has very weak commitment on the Mac and often lags years behind the latest releases and even then may have issues. If you make your bread and butter doing Java stuff, you should probably run an evironment that natively has first class support for it.
Of course, if your Eclipse use is just incidental then these may not apply.