Edit 2: After getting around to playing this one, yeah I agree, it gets hard fast once the counting mechanic is introduced. I got to level 19 and called it a night.
Edit: Sorry about the confusion. I was on the page for UFO 50 Nonograms and somehow switched to another game's page without noticing, and replied while under the mistaken impression that OP's comment was about the other game. My bad.If I may weigh in, I'd say that some of the puzzles are a lot harder than others, but nothing has been overwhelming for me. Over the years, I have played some truly ridiculous nonograms that took multiple hours to solve and the puzzles in this game come nowhere close to that (in fact I've solved every puzzle so far in less than 10 minutes each). One thing I appreciate is that the puzzles don't get bigger than 15x15, as while larger puzzles aren't hard per se, they can get tedious and exhausting, I've played many nonogram games where not even mid way through, they devolve into nothing but 30x30s and it's just obnoxious. I also appreciate that this game doesn't fall into the trap of dinging the player for making "mistakes", as that not only sucks the fun out of it, but also inadvertently acts as a free hint because the player can just reset the puzzle and use the information they got from getting dinged to their advantage. All in all, this is a very competent implementation. It's up there with Picross Touch and Pictopix over on steam. Very cozy. Very binge.
TL;DR I think the difficulty is fine where it's at. OP probably just got frustrated because the randomizer hit them with a harder puzzle that they weren't prepared for, i.e. the problem isn't the difficulty, but the difficulty curve. That can be smoothed out by manually assigning each puzzle a difficulty rank based on solve time and how stingy the hints are, and changing the randomizer so that the lower ranks are weighted heavier. Or just use a bag randomizer going through one rank at a time.