I don't know if you've played SpaceChem or if you're just saying that you've heard that it's difficult, so pardon me if I'm explaining anything you already knew.
Both games are puzzley in a sandboxy kind of way. You have a set input and a set output, and a huge degree of freedom when it comes to how you want to go about solving the problem. Unlike most games like this, where you have to solve the problem using a set number of tools, these games let you use whatever you want to solve the problem, as long as it gets done. It doesn't matter if your solution is horribly overcomplicated and inefficient, as long as you provide the required output, the puzzle is completed.
What's even better is that even though the game compares your efficiency to other players on steam with end-of-level graphs, the game itself doesn't score you. So even though your solution might be more inefficient than every other players' (and boy, did I have some colossal messes in both games), you don't have the game itself handing you one-star ratings or anything, so you never feel too bad about a suboptimal solution, and even the more clumsy and overdesigned solutions still feel great, since you spent hours on making them work.
This video does a good job explaining why these games are so captivating. Honestly, they've kind of ruined me for other puzzle games. Once you design the entire solution to a puzzle from scratch, it's kind of hard to go back to a game where you're trying to figure out the one solution that the developer intended, and hoping that you're thinking along the same lines that they were.
As far as the comparison between SpaceChem and Infinifactory goes (and yes, they both have cloud support), SpaceChem is definitely the harder of the two. Since you're not limited by what you can build with, the games have different limitations. In Spacechem, your machine will automatically fail if two atoms collide or if an atom goes outside of the playing area. In infinifactory, letting blocks push other blocks is a key part of the gameplay, and not only does a block falling out of the playing area not hurt you at all, but the areas are so huge (at least in the beginning) that it's practically a non-issue. SpaceChem feels fairly cramped in comparison, but that's where most of the challenge comes from. Not only do you have to figure out how to put together the molecule in question, but you've got limited space in which to do it.
