Moon druids are OP in the first few levels on the tabletop too, but quickly peter off around levels 5-9 as the martial classes gain extra attacks and better gear. At level 10 they get elemental forms and catch up a bit, but they're always a bit worse in melee and have to make up for it with their spell utility. Which they can't use in-form until level 18, so a druid always has to choose between retaining their limited transformations or using their spell utility to help the party. It is a pretty fun playstyle that strongly encourages planning ahead and is one of the harder classes to play.
That is kind of the niche of the moon druid. Massive amounts of health that allows them to tank better than any other class barring the barbarian, but they need active magic so that enemies aren't free to just ignore them and go after the rest of the party.
The main weakness though is two-fold; they have cripplingly low AC and no way to easily augment it and they have really low damage per turn unless they are using concentration on a spell to do damage for them. When I say low AC I mean VERY low. Bear form has an AC of about 11, and even their tankier forms only have an AC of 14 or so.
These two weaknesses play into each other. Because you can't cast in animal form for most of the game you are completely reliant on concentration spells you cast before transforming. If you lose concentration you CANNOT re-cast the spell without changing back. And because your AC is so low you are getting hit a few times every turn, meaning it is almost impossible to keep concentration reliably without investing feats into it.
Once we hit level 5 druid will be a lot more even with the other classes. Until then they get to be kind of OP, but not game-breakingly so. Especially since their OPness is mostly limited to being really good at getting hit so the rest of the party can have fun killing things around the druid.
