@Jabra:
Oh yeah, don't get me wrong - certain fruits are absolutely instant power ups to some degree. Mainly those that are already famous for being powerful. Like a logia-type is clearly useful, even if you feed it to someone like Caribou. That's why everyone wants them. Or the Gura Gura, where controlling the fruit doesn't matter if all you want is mindless destruction (fitting for Blackbeard).
But compared to the Raidsuit (the plus-10-to-all stats-thank-you-daddy-suit) there is room for improvement, room for effort, room for growth. Sabo (the prodigy, not your average dude) was able to launch a Hiken instantly. But I doubt he could use Ace' ultimate technique the moment he consumed the fruit. Why else would Oda bother to show him train the ability after all? You don't have to do that if it's really just about getting accustomed to the fruit. Else you could say Zoro is just getting accustomed to Enma, despite him training hard all these days in preparation for the war.
There is simply no room to grow with the raid suit, unless that growth involves Sanji asking his Nazi family for upgrades.
"Daddy, could you upgrade my raidsuit? We're about to fight the government!"
"Again? I already bought you the newest model last month!"
"But daddy, it's about Lebensraum in North Blue!"
"Oh okay, why didn't you say that earlier. Here's my credit card, also take some cloned slave soldiers while you're at it. Love ya!"
Granted, I'm not saying Sanji should eat the Mera Mera, it's still not what the Straw Hats are about (the people who turn shitty abilities into something amazing), but it would still be easier to digest for me than the raid suit.
But this applies to many abilities, albeit not as significant as Mera or Gura. Zoans are a no-brainer, they provide a physical boost and basically strengthen your already existing fighting style (you could argue that Sanji´s suit works like a Zoan really, only difference might be that you probably need to adjust your transformations, seeing Kaku, but that´s really all). And then there is Awakening of course, but only a handful of guys reach that point really. Logias similarly, they do not give a physical boost, but the intangibility will take you a long way, look at Caribou. Paramecia are a bit trickier in that regard, but the hax ones immediately give a huge boost.
I do not see that much difference between Emperor and Hiken, looking at how Ace was able to destroy several ships with one attack, and the island he used Emperor on was pretty small. Sabo is getting used to using it alongside his usual fighting style, he is not completely adapt to how Ace used it, but he is using it his way, hence you get dragon twist with fire, or Sabo using his speed with fire (no weight) to launch an attack, or kicking and so forth.
But i see no problem actually drawing out the power, which is what Crocodile first talked about, atl east that´s how i interpreted it back then.
Like i said, i still see it as akin to Zoan, plus invisibility. He gets the stats, he can still use his usual fighting style and it gets more powerful, he can still improve himself.
Besides transformation, like i mentioned, i do not see how you could train a Zoan fruit except physically, which Sanji can do now as well.