There are several generations of the Scarlett and two generations of the UCX, so it's better to be more specific.

The sound quality has been improved greatly in the Scarlett series over time, but there are some caveats still. But the UCX and even more so the UCX II are in a totally different league when it comes to features, flexibility, stability, support. UCX II is the ultimate audio interface in this form factor.

I think you should first define your use case requirements and your budget, as waedi said.

Thank you for answering!

Of course I know the huge differences between the two
in every aspect (price included),
and I used to have a Scarlett 1st gen which was not so good
compared to the next gens.
But now, the UCX is an enormous upgrade for my studio work.

I just wanted to hear what do you think about the UCX
if you ever had a Scarlett prior to that
(or any equivalent interface in that price range).

RME delivers the best you can get on the market at a still affordable price.
All the consumer devices are at a significantly lower quality level and that in almost every respect.

But even the competition with comparable or devices of higher prices do not come close to RME when you consider all aspects of a device including software, drivers, documentation, operation and features.

Because of the quality in all aspects and very long driver and firmware support, RME devices still have a very high resale value even as used devices.

But that should be clear, why else would you have bought an RME device, you ask that usually beforehand.

See information in my previous posting and link to blog article.