'Tis a simple language
After learning German, where each word has to be labeled as masculine, feminine, or neutral with der, die or das, Welsh appears quite easy really. It seems to me as though, roughly translated, most sentences are like the vocabulary of a small child, such as 'I want toy'. I think that makes it quite enjoyable to learn without to much stress on the grammar.
Mainly because the Welsh course is pretty simple? The language itself is not as simple as the course implies. Here's a taste of more formal Welsh: http://welshjournals.llgc.org.uk/browse/viewpage/llgc-id:1134021/llgc-id:1162341/llgc-id:1162355/get650
You can hardly compare formal Welsh to colloquial Welsh. Whilst I'm not a fluent Welsh speaker, I spent 11 years learning it in school and sat the 2nd language GCSE. Nothing felt particularly complicated with the language even at that point, beyond trying to remember what the vocabulary for everything was.
Read this and tell me how simple Welsh is: http://welshjournals.llgc.org.uk/browse/viewpage/llgc-id:1134021/llgc-id:1162341/llgc-id:1162355/get650
Hopefully you'll change your mind as you get deeper into the course - the end units are particularly challenging, I think. But it also depends on your stance: from the general of nouns POV, English is easier than Welsh and German, for example.
Also, there can be a huge difference between spoken and written Welsh; there's even a difference between the written form of spoken Welsh and literary Welsh. The Duolingo course reflects (or at least tries to) how people speak and not literary Welsh :)