User blog comment:TheRealD/Why won't Doflamingo uses his parasite ability on everyone in Dressrosa?/@comment-87.211.21.249-20141228184944/@comment-24850639-20150101234824

^ You're misinterpreting. "Limit" in this instance means things that the fruit just isn't capable of past a certain point. You're taking it to mean the user's stamina, or amount of energy. For example, Bartolomeo's barrier ability is limited to a finite amount of surface area. He can produce barriers of all kinds, but Pica's punch was just too large for him to produce a barrier that large. Has nothing to do with his stamina. It hasn't been shown, but more than likely Luffy has a limit for how far he can stretch. You can only stretch rubber so far before it breaks, or the built up tension forces you to let go.

As far as Doffy's abilities, I guess we just gotta wait until his fight with either Law or Luffy demands further explanation of his powers. At least as far as I'm concerned, I agree, there's just been way too many situational differences so far with Doffy's ability for us to really make heads or tails. He's shown varying amounts of concentration, and has used his abilities on varying level of strengths, so it's honestly just hard to say. For actual controlling of people, we've so far only seen him really do it on people that are weaker. Granted Stainless and the other guy  were Vice Admirals, we've seen Vice Admiral strength pretty much everywhere on a scale of 1-5. He's been shown to be able to "restrict" people effortlessly, but that seems to be when he intends to actually fight. The people that he's controlled have been shown to be unable to resist, so honestly Luffy breaking free from his string cuffs to beat the fake Doffy are the first time any form of resistance to it has been explicitly shown.