Pros:
+Colourful cast of characters. In games like this I find that having a varied-looking roster is a big deal, and I'm happy to say that everyone in this game have their own unique look and personality about them. Some (all?) of them even have unique traits, although these seem to be minor, like a character who does an auto-side step when he blocks an attack.
+The variety of different arm types. I didn't know enough about this game before going in, so the fact there was so many different arms surprised me. They each have unique properties, and for a game called "Arms", they did a great job of making the arms feel like different weapons.
+The stages are another thing that I didn't realise were so diverse. I figured it'd just be big empty boxing ring-like stages, but there's quite a few nice looking ones with their own gimmicks, like those beyblade-type things.

Cons:
-Very light on content. Even with DLC there's only 15 characters and stages. There's no story mode and arcade mode is basically the exact same for each character. There's a couple of extra modes, like 1v100 or the 3 types of mini-games, but they feel like nothing more than a few minutes of distraction.
-Control customisation is severely limited. I had problems with both the motion controls and the joycons in grip controls. In motion controls I found moving with motion control to be a pain, but you can't change it to the joystick, and in the joycon grip the block button is in an awkward place and you can't change that either.

Mixed/Not important enough to be a pro or con:
~So I haven't unlocked every arm for every character, but with what I have unlocked, it looks like every character can actually unlock every arm? On one hand it's nice that they basically let you use any character you want without worrying about how good their arms are, on the other hand it does have a huge impact on how unique the characters are.
~The game clearly emphasises motion controls above any other control scheme, but to me it just felt like movement with that was way too slow. I can't call it a complete con because it does let you use the controller, but I can't call that a pro either because when you do use the control you can't help but feel like you're not getting the full game experience (and I'm pretty sure the control you have over the arms is limited when you play with a controller too, since you can't control each arm separately). So even though the game gives you some choice, it felt like no matter what I picked I was missing out on something. I think that if I was fully invested in the game (I'm only playing it as part of the free trial for Nintendo Online members) I would put more effort into getting used to the motion controls.

Notes:
•I haven't played a ton of online, but what I did try out was super smooth and fun.