CoS with 5.5e rules.

I've heard some things online that the Curse of Strahd is worse with 5.5e rules than 5e ones. (And i've already stated my thoughts on Strahd's new stat block...) Have any of you run that campaign with the new rules, if so, how did it go?

I've run it for just two groups. To get it out of the way my bias here is that I genuinely feel like 5.5e is more focused on casual games and one-shots than full campaigns. So take what I say with that context.

My players across those two groups have found the module trivially easy. Group 1 is a group of GMs who get together to test a load of different systems and suchlike. Group 2 are players who've spent years with 5e, 4e, and aD&D. Here are some of the complaints these two groups had.

- Starting with the biggest issue of all - the 5.5e Strahd stat block (From the newest Ravenloft Book) is a complete joke which undermines the horror theming. He has fewer spells, a weaker grapple escape DC, weaker damage (compared to the damage against 5e characters), lost his Children of the Night summon, weaker Legendary Actions, and loses Lair Actions
- Bonus action potion use makes Barovia less scary. Because it undermines action economy and makes healing significantly easier this rule alteration isn't ideal for the setting. A few of the players suggested that 5e potion rules should be required in Barovia.
- Background pivoting from largely RP (i.e. non specifically combat) focused things to having more specific and tangible combat effects with the origin Feats made two players feel like 5.5e supported less roleplaying not more. (Not CoS specific here, but it came up as a result of a CoS game).
- There's no updated Madness effect tables for 2024 that we were aware of at time of play. 
- Regaining all hit dice on the Long Rest meant that there was no attrition effect and as a result the adventure felt less dangerous or scary.
- Being able to explicitly cast cantrips during a short rest again made the adventure feel less dangerous.
- All enemies felt significantly weaker, especially named monsters (basically those not in the 5.5e Monster Manual).

I can imagine you're seeing a theme here. Mostly the players wanted something quite challenging and 5.5e didn't feel like it is a challenging system. As a result it made the adventure feel significantly easier. Not want you want from a proper horror focused adventure in my opinion. 

The consensus from those I've played 5.5e with is that it's a system designed for those who want to be able to say they've played the more challenging adventures, without actually experiencing the challenge. It's for those who would prefer to turn the difficulty down a little...which is absolutely reasonable. However, that means 5.5e doesn't land for every player out there with 5e designed modules. Again, I'm biased against 5.5e so do take what I say with a pinch of salt and make up your own mind.