not a lot really happens? she goes to join the fight, training montage of non events, she causes an avalanche, gets send home, returns at the last minute.
The overwhelming majority of Disney films thrive on a compelling villain (who gets a dope song). The invaders were empty plot devices, without clear motivation, have no personal relationship with the heroine, or really much connection to the narrative thrust or character beats.
Internal and personal conflicts are great and meaningful material, but the Disney Animated Film is a specific genre, which broadly speaking Mulan doesn't really match. It doesn't even mean it's a bad film, just that it's being judged by a rubric it doesn't compare to.
It also has some weird tonal stuff. Mushu as comic relief doesn't work very well. I know they were trying to bottle Robin Williams Genie lightning, but what they missed was that Genie was a central player in the story, and interacted back and forth with a witty protagonist. Mulan is kind of self-serious, and Mushu is a greek chorus, not an actor, so there's no space for him to actually do anything. As a result, all of his energy, while great, ends up sucking energy from the stuff that's actually moving the story forward, which is not a great decision.
Ultimately, it doesn't really do everything the makers set out to do, which itself produces something still very interesting. It's technically fantastic from an animation standpoint, has some truly great songs, and handles serious themes in a thoughtful way, which is in some ways much more interesting than if they gave us "Beauty and The Beast 3, this time we're Chinese" but we're still left with actual flaws to talk about, which means a review of it sounds worse than it actually is.
tldr: "a solid movie with a couple flaws that's worth seeing" sounds like condemnation when we start talking about it's contemporaries from the same studio being the best animated films ever made.