Regarding subclass comparison, as it's own thing:
Spore druid is the "Forget about wildshape. I don't care about being an animal, I just want to cast druid spells as a caster character. I'll being in humanoid form at all times, so please give me something else to do with my wildshape charges so they aren't a waste." subclass.
And so it does. Spore druids aren't really meant to use animal forms in the first place, so talking about how effective they are is a bit of a moot point. If you're an animal as a spore, you're doing something wrong. Change subclasses if you're gonna do that.
Moon druid is the "Screw spells. I don't care about being a caster, I just want to be a beast. I'm going to be in animal form at all times, so please give me something else to do with those spell slots so they aren't a waste." subclass.
And so it does. Moonies can use spell slots to heal themselves in beast form so players don't feel like part of their build is being wasted by never being in human form. Not to mention the healing actually keeping them in the animal form instead of getting reverted by enemies. If you're in humanoid form casting spells as a moonie, you're doing something wrong. Change subclasses if you're gonna do that. And appreciate having less competition in your party for the best items I suppose.
I honestly don't remember what Circle of Land gets to differentiate them from other subclasses, but presumably they're the hybrid subclass for actually using both your spells and your wildshape.
So if you intend to use both druid features, to do things with spells before wildshaping, use either at different times, or charge in in animal form until knocked out of it and be a caster after, go land druid. The other subclasses are for picking one of the two, not doing both.
Yes and no. It's true that majority of druid circles are primarily casters, be it land, spore, wildfire, etc.
However, saying that Moon Druid is in animal form all the time is a bit missleading. On levels 2-4 they are probably the strongest class in game, due to having basically tripple hitpoints and spells on top of that. Their spells at that level are pretty bad though (many of which don't work in BG3 like the Moonbeam). And their progression really falls of after level 5 up to 9. At that point martials get double attack, smites, extra feats like Sharpshooter, etc. At that point, casting spells and then maybe thinking of Wild Shape is pretty standard. Case in point, Conjure Animals. It lets you summon 8 wolves, which are far better than wildshaping into a bear or something. Even at level 6 turning into CR 2 creature isn't that important. And since Druids get a ton of concentration spells, they often end up with plenty of spell slots with no way to use them, outside of maybe Lunar Mend, which is objectively worse than healing word or cure wounds. Still conjuring a small army and then running away from enemies is a valid strategy for Moon Druid. And at level 8, you can cast 'Call Lighting' turn into a bird and stay out of enemy's range, dealing damage to them from distance.
In fact, once you reach level 7, you can cast Polymorph at your ally and turn them into a Giant Ape, which is amazing due to high damage, high hitpoints, etc. However, you shouldn't be casting on yourself, as its worse than Wildshape in many ways. Your saving throws, proficiencies, etc don't carry over in polymorph. You can also use spells like Erupting Earth to deal damage and create difficult terrain, etc. As you get more levels, your wildshapes are left behind, while your spellcasting only gets stronger. Of course make no mistake, it still serves purpose, as Wildshaping often allows you to survive major damage, as Druids are known for their less-than-average AC.
That all changes at level 10. You can turn into a CR 5 elemental with resistance to all non-magical damage. That's incredible. To put into perspective out of about 2000 monsters in DND, that are not NPCs, only about 60 of them have got magical weapons, that's about 3% of all the monsters. So you can cast powerful spells, while being incredibly hard to damage and being able to fight in melee.
However, BG 3 doesn't let you do that for several reasons:
-PHB only, for the most part. Druids with only the basic book are really gimped by the lack of good spells. They don't get Absorb Elements, Summons Beasts, Erupting Earth, etc. Most of their spells there are concentrations and regardless of which subclass you take, your basic spells barely matter. That's what makes land, spores, wildfire competitive. They get extra spells Moon Druids don't.
-Primal Strike is bugged. It has been bugged since release and still wasn't fixed. Act 2 really hurts Moon Druids because of that.
-Many interactions, class features, feats don't work in wildshape or weirdly, there is a whole list on reddit.
-Magic weapons are basically everywhere. All gith have got magic weapons, later enemies have got magic weapons, etc.
-ALL Elementals don't have resistance to non-magical damage, which would still suck due to reason above.
-Enemies having immunity/resitance to physical damage in general. There is a boss in Act 2 that is completely immune to ALL physical damage, no exception.
-Due to massive power creep, Larian buffed wildshape to compensate (owlbear at 6, Myrmiddons at 10). However, Moon Druids don't get almost any items that work (as per RAW, with few exceptions), while Martials can hit like 19 times in one turn with fully Min Maxed build.
-Fixing Moon Druid isn't a matter of fixing just 'Primal Strike' the whole game is so full of broken magic items, combinations, etc that makes most casters a bit weak, with the exception of Sorcerers due to their Metamagic.
Overpowered Build number 1TLDR: Moon Druids need more than just some bug fixes to be competitive.
