I don't think Maokai required a rework, well now it cannot be changed but what can be changed is that the speed of the ultimate should be increases twice than that in the public servers because it is really hard to land it as it takes forever to reach the intended enemy.
You're not using it right. I find that it's actually a really good ult, you just gotta know how to use it.
You have a few options.
Flank them, ult them into your team. Either they take the root (which at max distance scaling is a 2.4 second root) or run into your team.
Drop it, and charge using Righteous Glory to get in range for W, then Q them back toward it. Mao wants a Catalyst item, and Glory's 500 HP makes it a sweet purchase, especially when you synergize it well with ult. If you are so inclined, you could have Mobi-boots as an alternative (Although one of Mao's weaknesses after the rework is auto attackers, so dropping the Tabi is a bit hard of a sell).
Counter-engage. Drop it as the enemy team is coming at your own, preferably a bit offset to either side before casting it. The enemy either backs off and likely gets split, or has to drop their engage entirely. Doesn't mean they won't get snared either if you cut them off right.
Chain with Ally CC. If you have any allies with a long enough duration CC, you can catch them with the tail end of the ult after they get CCed to keep the CC chain going for an extra 4 seconds of snare/root.
And here is how you don't use it.
Chasing an enemy. Almost never will it catch an enemy without massive extra aid from the rest of the team in the CC department.
Against a high mobility target to try and make a pick. They will laugh as they just run away.
Here is a few great examples of using Mao's ult correctly.
Ganking bottom lane, go behind the enemy tower, and ult toward the lane, leading them to either commit to a 2v2 as you come in to make it a 3v2, or run into the ult trying to get past you and getting caught by the snare, or trying to run past your laners down river, which is heavily unlikely to succeed especially if you have your mid come down river.
Your team is doing Dragon/Baron, and the enemy team tries to collapse on you from both sides. Ult down one side to catch them in the snare, and jump the other in a 5v1, 2, or 3 in your favor in what should have been a flanked 5v5.
Your team has a Bard. He lands an ult a short to medium distance away, and you pop your own toward the enemy (From a fairly close distance). The ult can accelerate enough to run down a team after the first few seconds of charging up speed, but it needs to get that speed up for that to happen. Either you snare them as they are coming out of the stasis, or even better yet they may run for a moment more and get caught by an even longer snare
I could be way off, but I think with the speed of it, it's not actually meant to be used as a straight up engage? I found it's good for ganks, if you sandwich the enemy and have the laner on one side and the wall of roots on the other, it's a bit of a bad situation for them. But mostly I ended up using it for a disengage. Like, oh, they're chasing us? They can't chase us through the rootwave.
Maokai did require a rework because his old ult was major balancing issue. He's been getting more and more nerfs ever since late season 4 (if you look, vary rarely do you even see a buff, if any in his balancing history since then). The problem with Maokai's ult was that it was an AOE Damage Reduction Nuke. It allowed him to be extremely tank as long as it was active and anyone else within the zone. After so long you could pop it to deal a good chunk of damage. Because of the non-damage portion of the ult, the rest of his kit had to suffer. With this new ult, they can now give Maokai power into his kit which they have done with the rework.
I think his ult's speed should scale with his movement speed now personally. He might be a tank, but tanks build Dead Man's and tier 2 boots so it would be viable.
