Atom RPG is a good game, but it's a bit simplistic at times. Don't get me wrong, I think it's a must play for any RPG fan, but it's short and a bit strange in terms of story.
Asides from that, it has great progression, an almost bearable trader system, lot's of places to explore with side quests that are fun. I actually liked enough to buy their next game, Trudograd, in early access for 10 euros. It's halfway done, but the 10 hours they have out is pretty similar to ATOM RPG (you can even use you old character) so it feels like an identical sequel to ATOM RPG in terms of mechanics, just a different location.
Underrail is awesome, but it did feel a bit contribed at times, and many quests are dependant on prior requirements. I also don't know how to feel about how 80% of the builds yyou make are wrong and should be ignored, since the game allows you to fuck up big time, but won't let you truly find out until you have 20 hours into the game. It does have a lot of skill checks and it feels rewarding to put points into a skill to see how the game will respond to you.
In conclussion, UR is better than Atom, but the increase in difficulty and the unfriendly systems of UR make it a bit tougher than it needs to be, so I would play Atom first if I'm looking for a Chill, entertaining RPG and then, once I'm sweaty enough, dig into UR.