The tapered threads of some conduit bodies are not intended for use with EMT fittings,chase nipples, or any straight thread fitting, just IMC/RMC, and some are not listed for use as explosion proof fittings.

http://www.appletonelec.com/pdf/A-2-3-4-5-6-15.pdf

http://www.o-zgedney.com/PDF/OZG_AB%5B1%5D.pdf