It all depends on what cards you look at. For example, the Fury X has almost no overclocking head room, while its competing card, the 980ti can be OC'ed a ton. On the other hand, if you look at the HD 7970, it had a ton of OC head room, while the competing GTX 680 had a bit less OC ability.
You have to look at specific cards that you are comparing to make a claim.