The X-Men TV-series from the 90ies is a strange beast in that most of the writing/characterization is clearly aimed at a teenage audience (best example would be Wolverine, written as a lovesick and grumpy puppy, not a stone-cold killer), but most of the plots contain elements you would not have seen in any other teenage series at the time. Due to the limitation leveled at the creators, no bullets were allowed (all weapons were some sort of energy weapons), yet at the same time the Days of the Future Past arc showed graves of most of the main characters and death and killing was openly discussed.
Sure, even shitty mature series at the time had no problems doing that, but you got away with far less in what essentially was a kid show, and for that it was dark and mature beyond what most of its peers did. The main arc of the first season dealt with a killer robot program to eradicate mutants and the introduction of various famous elements from the X-Men canon, like off again-on again main villain Magneto who tried to save the mutants by killing all humans (sort of).
When I first watched this I was part of the target audience and loved the series without hesitation, now the flaws are far more obvious to me and some of the writing is really grating and the animation looks decidedly less impressive. That said, the first season covers a lot of ground from the X-Men canon (Days of Future Past, Apocalypse rising) and despite all the flaws its still fun to watch. Not perfect by any means and some of the later animated X-Men series were much better in comparison, but it’s still fun seeing the creators testing with what they could get away with.