Both EMT and FMC are allowed to be run concealed. I can't believe that the fittings (couplings and connectors) would have to be accessible, even if there is a transition from one raceway type to another.

After reading the commentery in the Handbook, I think 300.15(F) only applies to cable/raceway transitions.

Few things are harder to put up with than the annoyance of a good example.