We aren't using such a service yet at any of the established businesses I work with but the intent is to shift at the earliest reasonable opportunity.

Migrating is not trivial if you have an existing customer base who are already signed up via other payment service providers. A merchant of record will typically need all end customers to sign up personally and thereby enter into a sale contract with them and not with you. That means you can't just transfer existing payment details and authorisations the way you can between different PSPs processing card payments for you. And if you don't migrate everyone, you still have to handle all the relevant tax admin anyway and lose much of the benefit of working with a MoR.

So the new SaaS I was working on was planning to use a MoR from the start, while the established B2C business will probably wait for convenient opportunities like launching new products and retiring old ones to make the shift to minimise churn.

I don't know if a final decision has been made about which service the new SaaS will use. Among my personal network Paddle has had positive feedback and I think that's probably what that business will end up going with. I know there were some concerns about the legal arrangements and Paddle's terms and I don't know if they've been addressed yet.