You're stuck with cloudflare nameservers¹, so if you want to change nameservers you need to transfer them to other registrar, how much of a deal breaker this is is up to you, to me is project dependent.

1. Section 6.1 of https://www.cloudflare.com/domain-registration-agreement/

I've helped move a few domains from Gandi to Cloudflare. The move was relatively straightforward (couldn't get the Gandi records export to import into Cloudflare so had to do it manually...), and the new domain and renewal prices are lower.

To replace Gandi email (that Gandi went from free to ramping up the pricing for when they were taken over), Cloudflare offer email forwarding so you can receive incoming mail from a custom domain to e.g. a gmail account, and for sending mail you can pair this with a custom SMTP service like https://www.smtp2go.com (1000 emails/month on the free tier).

Apart from that, DNS is something I barely touch for years sometimes so I don't find much difference between registrars beside their pricing and how much you can trust them.

Being able to point your Cloudflare nameserver records would be nice though, so worst case you'd need to move everything if another registrar had some services you were interested in? Would be curious to know more about how common this scenario comes up and why.

I’ve got about twenty domains listed with them. No problems after 2-3 years, but I do wonder whether it’s a good idea to use the same vendor for DNS and domain registration.

I am happily using them for all of my domains they support. The problem with Cloudflare registrar is that they flat out don't support many domains/tlds.