It's a low quality movie that I remember watching as a kid and being up in the night thinking about it because of how much it scarred me.
*plot spoilers*
The movie was scary as a kid.
The way the mom is kidnapped and almost dies to suffocation, the way the martians look and behave, it's all just creepy and unsettling and after watching it I was left feeling uncomfortable. Not to mention the boy in the movie, Milo, is young (around the same age I was when I watched it) and the concept of being left alone like in a foreign place with no way to get home is terrifying.
Another thing, everyone is hating on the animation and while it isn't great by any means, it isn't unbearable. However, it wasn't very colorful or aesthetically pleasing, besides maybe one scene where they are in a cave, otherwise the backgrounds felt bland, the character designs were weird, and the movement felt dull.
The movie also has you on the edge of your seat, but not in a fun way. More of a "I can predict what's going to happen, but it's taking a while to reach that point" way, if that makes sense. It's anxiety inducing.
Lastly, there was a plethora of plot holes. Such as the kid understanding how the clocks work after the most minimal explanation, or the way gravity changes to make it more intense.
I dunno, it's just not a movie I'd recommend to anyone.