Well, FPC's default mode is not delphi compatible...
But, since I don't use objects and/or classes, there are only very few instances when I need to write code that is specific to either compiler.
The only time I code to FPC is when the program is going to be 64bit only, otherwise, it's fairly easy to make both compilers happy with a single source with just a very few "$ifdef" to keep them happy (very often, just one small group of definitions are the beginning of the source in a single "$ifdef".)