I've been saying that for decades, here and on DP/Whitesnake boards!
Cozy was a lovable character and his (inherently limited) "barbarians at the gate"-style fitted music with a certain grandeur well, not just Rainbow and Sabbath, but also here (me thinks)
but he couldn't swing like Ian Paice to save his life. These track sums him up, Cozy was a stomper, it's what you got from him ... I'm like a freight train ...
Moreover, Cozy rushed live like hell, Blackmore said he was "always ahead of the beat, while Ian Paice was 'the beat'" (Blackmore loved his energy though), Bob Daisley said he "cut corners all the time" (and that it cost him the job with Gary Moore who wanted a metronomic drummer) and Tony Carey is on record for saying that "Cozy speeded live often so much, it became hilarious".
Neil Murray has said this about him:
Can you explain what made Cozy such a unique and talented drummer?
Well, his drumming was an extension of his personality. He was a very fun guy a lot of the time, and very bright, intelligent. But he had a very forceful strength of personality. He would very much be a driving force in terms of his participation in any band that he was in. Even if he wasn’t the leader, he’d be almost the co-leader. He’d want to be seen virtually as important as, let’s say, David Coverdale or Tony Iommi. He didn’t want to be just a backing musician. He’d get very frustrated if he was expected to be just in that position.
It carries over to his drumming style. He had people that he really admired, and wished he could emulate, like John Bonham or even Jon Hiseman and other jazzier drummers in terms of technique, Jeff Porcaro later on. He was comfortable with his own style: “This is how I play.” In a way, it’s great to play with somebody like that, who is so sure of what they’re doing. With the massive sound he had, he just hits you right and you try and make it even stronger.
With somebody who is so powerful a presence, it means that, possibly, the bass player has less opportunity to shine. You’re more kind of in the background, but it’s still a very satisfying combination to play with somebody like that, someone very powerful that can drive the whole thing along.
But even Neil who played as Powell's bass foil in Cozy Powell's Hammer, Whitesnake, Black Sabbath, Brian May (who also loved Cozy's no-holds-barred energy) and Peter Green's Splinter Group had to adapt (= dumb down) his bass playing to be compatible with Cozy's style.
And that leads us to Colin Hodgkinson, the bassist who was both Neil's successor and predecessor (once Cozy had joined on drums) in Whitesnake. Hodgkinson with his jazzy background couldn't gel with Cozy at all. At one point, Cozy (who had a temper) rushed out of the drum booth at the Slide It In sessions snapping at Colin: "Let me introduce myself, I'M THE DRUMMER!" I saw Whitesnake with Colin a few times, every time he looked outright bemused and even bewildered on stage, in fascinated incomprehension about the powerhouse that was Cozy Powell. And while Cozy might have clobbered early Whitesnake material to death, Hodgkinson was also a culprit as he had not the faintest idea what and where to play within Whitesnake, he was totally out of his depth (Jon Lord had recommended him to David Coverdale after having toured with him in Germany playing blues standards in clubs with Chris Farlowe, Miller Anderson and Pete York, an old-school swinger of a drummer - it had nothing to do with playing Whitesnake material in large halls alongside Cozy Powell). Hodgkinson has a style all of his own and is a fine bassist, but he's quaintly angular in his playing and his middish sound is anathema to what a bassist needs to provide with Cozy.
Here's the otherwise exactly same track once with Colin
and rerecorded by Neil on bass:
Listen to what both of them do around the 2:15 mark, after the guitar solo, the difference is subtle in actual playing, but the difference in feel is night and day.