Initially just canvassing opinion/"putting it out there..."
So far I've been delighted with my 7D Mk II - it improves over my 7D and 70D in every area that matters to me.
Whereas those cameras both work perfectly well with my Sigma 120-300mm f/2.8 and 70-200mm f/2.8 lenses attached and a 2x TC (Sigma or Canon Mk II) between them, my 7D mk II over-exposes by almost two stops with the same lens/TC combos.
In any exposure mode.
1.4x TCs from Canon, Sigma and Kenko work perfectly with the 7D Mk II and the SIgma lenses; but with either 2x, exposure is all to hell.
I can get round this easily enough by dialing in -1.75 EC, but it seems clear enough that this is something that needs to be addressed - presumably via a firmware update.