This class is well put together. It has nice art, seems to understand 5e (outside of some grips about it's use of floating modifiers), and will not instantly break your game. The concept is cool, and clearly a lot of work was put into it.
The problem with this class is not necessarily that it is too strong, but that it is too hard to challenge. I think the balancing factor of it is that it does not do all that much damage (though you can get good damage from it). The issue, and why I won't allow it as an option for my players, is that it Mist Walk just serves as a trump card to too much content. Due to how it is written (not using the creatures movement speed) it has some less obvious power such as eliminating the movement penalty of halfings and dwarves to some more drastic effects like making the Mist Walker essentially immune to prone, grappled, restrained, etc.
These guys are going to sail through infilitration, traps, etc. They make a flying race look like a chump when it comes to the non-combat pillars of content. Doors, traps, pits, Walls of Force... there is no obstacle that will slow these guys down. That is going to heavily impact how you have to design encounters. Something like a Roper is just entirely ineffective against these guys. Just think of them as have permenant always on Misty Step, but better. Especially when you get to higher level and the distance they can travel gets absurd, you will just never be able to present a meaningful obstacle to a player with this class, and they won't even have to spend any resources to do it, and to me, that's a deal breaker.
...and we should probably talk about that they get that at level 1, so... basically everyone, ever, is going to take a level in this class if you do multiclassing with optimization monkeys in the parties. I don't know if I hold class designers responsible for mulitclass, but my inclination is yes, and this is pretty ridiculous on that front. It should also be noted this is a high complexity class, sort of like the UA Mystic (though not quite as bad). In play as a DM you'll have to keep the class document next to for the "you can do what??" moments that will come up with fair regularity.
I think it's definitely interesting. There's a lot of cool stuff here; I enjoyed reading through, and I like a lot of the people that worked on it (I follow Cody's YouTube channel) but, ultimately, I can't recommend it as a class that you can use, as, personally as a DM, I'm not going to add it to the list of classes I let players pick from, so its just something interesting to read through.