I just popped on my Kenko extension tubes on to my 5D Mark III and they work just fine. I bought them about 2 years ago.
Extension tubes have no electronics, they simply pass through the signals between lens and body. There can't be a compatibility issue with extensions tubes. I suppose if you stack enough of them there may be an increase in electrical resistance which could interfere with the signaling. This is not what is being discussed with regards to teleconverters. They have a cpu in them and intercept/modify communications between the lens and body making the process potentially prone to incompatibility.
5D Mk III, Elan 7e, Samsung NX 1000, some old rangefinders
Tamron 24-70 2.8 VC, Canon 70-200 f/2.8L IS II, 400mm f/2.8L II, 100mm L IS Macro, Sigma 85mm, 50mm, & 35mm f/1.4's, Rokinon 14mm f/2.8, Canon 2x Extender II, Kenko 1.4x, 430 EX II, Elinchroms