Ukrainian is a lot easier if you know some Russian.
I used to do the Ukrainian tree until the Russian tree hit beta but I really struggled with it. I recently decided to do some Ukrainian again and was pleasantly surprised by how smooth it went. Knowing a bit of one slavic language really helps with learning the others!
I recommend doing the Ukrainian tree after you have either finished the Russian tree or made enough progress. The Ukrainian tree is short but unpolished. Many grammar notes are still missing and some grammar mistakes still haven't been fixed. If you solely want to do the Ukrainian tree: Take your time. Take a short break when lessons gets frustrating. ( Watch some videos in Ukrainian then).
Ukrainian is a beautiful language worth picking up if you dig slavic languages and/or Ukraine. Ukrainian learning materials are pretty scarce so having a duolingo Ukrainian tree is pretty neat.
I've heard that's learning Polish even more helpful than learning Russian ;) According to Wikipedia, Ukrainian is 70% similar to Polish and 62% to Russian by vocabulary. Also it shares 22 gramatical and phonetic features with Polish and 11 with Russian (the least in whole Slavic group). For me Ukrainian is rather easy, but I'm an absolute begginer ;) But every Slavic language is good here :)
I think that it's more similar to Polish in vocabulary, but more similar to Russian in grammar (and the letters of course)
We could also learn Polish instead of Russian.I've also heard of the similarities between these two languages
I learnt Russian years ago. Even though my Russian knowledge is quite rusty, it helped me a lot with Polish.
It is certainly correct that already knowing one Slavic language helps you learn the rest of them - the grammar all operates under the same logic (even if the endings etc are different) and most of the vocabulary is recognisable as cognates.