I am also a big REAPER fan.
Biggest cons I can think of:
1. Stock FX plugs are ugly. Not lacking function in any way, just ugly. That shouldn't bother any serious producer at all.
2. No real inbuilt instruments or samplers. Yes, there's reasynth and reasamplomatic 5000, but they're not exactly powerful. You have to use your own VSTs, which is what a lot of producers do anyway, even if they're not using REAPER. With the money you save from buying REAPER you could easily afford a better synth than any of the stock plugins in any other DAW.
3. Slightly morbid name, connotations of death.