Makes sense, but I'm not sure how many people are suited to answer. If someone plays a lot of no increment for example, they're in a sense specializing in it. They'll be worse at games with increment and similarly an increment specialist will be worse at something like 5|0.
For example lets say my friend and I are both 1800 OTB and both 1800 at 5|5. But my friend plays 3|0 a few times a week while I don't. I'll have a lower 3|0 rating, but only because I'm inexperienced (or perhaps I have less dexterity, or mental quickness). In this case would you say 3|0 is objectively harder, or just that as an individual it doesn't suit me?
3|2 mins VS 10 mins VS 5|5 mins VS 3 mins VS 5 mins
Stronger the pool of players, the more deflated the ratings are. For example, if only OTB grandmasters played online, half of those grandmasters would be rated under 1200. If only absolute beginners played, many of those beginners would have ratings greater than 2000.
Which has more high rated players? How a 1800 (for exmaple) rated blitz player who only plays 3|0 compares in chess ability to a 1800 rated blitz player who only plays 5|0. Obviously the difference won't be great but I think it's a very interesting question, one that I have wondered before (a variation of).