I've been alternating between "normal" and "slow", and I find "slow" much better. There are no game-breaking issues for me that are introduced by playing on "slow" (and in the thread
@PRO_TOO mentions above, you can see people reporting the CBs "switching off" as often when they play on normal speed).
What I
am finding is that games on World Class are much more end-to-end, with 20 shots per game on average, whereas games on Legendary are much tougher (seemingly for both sides), with 12 shots per game on average. (Ultimate is alien-football garbage.)
What I don't know is if lower-league teams play like gods on Legendary - I've got Cambridge (as Burnley) in my next career mode game, so I guess I'll find out tonight...
It's just such a shame that we don't have
intelligence sliders. I want the less ping-pong-passes from a lower difficulty, but the difficult-to-break-down midfield from Legendary.
Either way, I'm getting absolutely smashed on any difficulty I play.

But in previous years, I felt like that was because the AI was ping-ponging the ball around and all I could do was watch. This time around I feel like it's more because of my terrible defending.
Oh - and the other thing I've found is that an acceleration drop of a single point for the AI has a fairly huge impact. Playing as Burnley against Liverpool on World Class, I found that with 50/50 acceleration, I was 3-0 down by half-time - but with acceleration at 50/49 in the second half, they couldn't score.
Which makes me worry that pace is too important (but I just hate the silly insta-dribbles, and this cuts a lot of them out).