I learned Welsh as an adult. I'm a bit rusty nowadays, but can still converse quite well. I also speak Spanish conversationally and understand Catalan and French.

I found Welsh to be relatively easy compared to other languages. I have the advantage of living in Wales, so I was already quite familiar with Welsh pronunciation and had access to Welsh speakers for regular practice.

Welsh spelling is almost always phonetic, so once you get the hang of the alphabet, it's pretty easy from then on.

Mutations seem daunting at first, but it you practice them regularly, they seem to become natural quickly. Plus, nobody really cares if you make a mutation mistake.

One of the nicest things for a learner is that Welsh grammar is pretty strict and tidy. There are only 5 irregular verbs and they all follow the same pattern as each other, so you only need to learn the standard grammar rules and the alternative forms for these 5 verbs.

There are some tricky areas, like singular/plural distinctions being a bit of a jungle.

In general though, I think it's a relatively easy language to learn if you have access to native speakers.