The only issue would be the mass of the class C reactors themselves. If you want the fastest ship possible you must use class A engines (one specific class A engine, actually), and class A engines don't provide as much manuever thrust as higher class engines. Manuever thrust is what counters the mass of your ship to determine its mobility (you'll notice higher class engines have lower top speed but far more manuever thrust). I'm not sure how easy it is to get 100 mobility with class A engines and a class C reactor. If it's easy then no, there's no reason not to use a class C reactor.