I bought a Kenko one a few weeks ago, the PRO 300 dgx for £99.00 from speedgraphic, i fitted to the lens and body, it would take one shot then the vf would black, disconnected and cleaned contacts etc...tried again and still the same issue. when releasing the lens and tc from body i could hear a release type of sound as if something was snagging. I sent back and managed to get a mint nikon 1.4tcii of here, pleased to say its working fine.
Edit: forgot to add, Rich u8myufo, also had problems with the Kenko and D750, which i have.