After playing OU for a bit too much, I decided to try UU for a change and ladder for the first time since early ORAS. I might not be as experienced as others here, but I do think I got a good feel for the general idea of the tier. I took a look at some of the snake games after the Jirachi ban before laddering to see what worked and what didn't structure wise, and then built 3-4 teams until I found one that I felt comfortable with, which was a very standard VoltTurn one. I peaked 23 on the ladder until I ran into stall 3 times in a row and went on tilt, lol. Regardless, hopefully this is valuable input as someone who didn't know what was broken 2 weeks ago.
Ultimately, the problem is that Weavile's combination of strong attack + utility in Knock Off and Pursuit are problematic for the majority of the tier, especially when paired with VoltTurn. It's very easy to mention running something like keys or another steel to prevent Weavile from being problematic, but I simply don't see how this would help the meta, rather make it more stale than it already is. As it stands, it's not fun to play a tier which is infested by predominantly two strategies: VoltTurn and spike stacking. I might be regurgitating information already posted, but I wanted to look at what was mentioned below and actually give some examples that I encountered while laddering.
TL,DR
The 50/50 traps don't happen that much and when they do the Weavile user is at risk and even if successful it doesn't guarantee a sweep. The fact that teammates help increase Weavile's power shouldn't be an argument for its brokeness, its an argument that it needs support. And Weavile being banned won't help the Volt-Turn issue because Scizor + Rotom Wash have perfect synergy and top tiered viability independent of Weavile.
Even if said 50/50's don't happen as often as stated on paper, they still do happen, and I ran into the bottom 3 multiple times:
ex 1) My opponent, even with a steel type (usually Cobalion) in the back, would have their Latias out vs. my Weavile. I do think that what pif points out in that the Weavile user is at risk is true, but I don't think this is enough to counteract the unhealthy 50/50 present. If Weavile happens to be LO instead of CB and you correctly switch out on the Icicle Crash, for example, then you have to be weary about Low Kick because it will proceed to kill Cobalion. Now, you have to switch something else in to scout for Low Kick and risk another move coming your way (i.e you go Rotom-W while they Knock), or simply stay in hoping they don't click Low Kick. This is all risk/reward, but I don't think that these types of mindgames where one pokemon makes you have to scout several times is not beneficial to the meta and further constrains teambuilding.
ex 2) A lot of the times, my opponent would have their Mega Garde out vs my Weavile. You can already see the 50/50 in play here, but there is something deeper: I usually ran into these scenarios when playing against opposing VoltTurn teams (trying to get in Mega Garde healthy), and the moment you make an incorrect prediction because of Weavile and/or lose a mon, you're already losing the VoltTurn momentum battle by being a mon down. By losing the momentum battle, you find yourself in a very hard situation to dig out of, as your opponent will always have one more mon than you and can VoltTurn out into their breakers.
ex 3) I usually used Infernape as my scarfer of choice because it outspeeds most other scarfers and provides U-Turn momentum as well. Mons like Latias and Chandelure, both of which I ran into a lot and are problematic for Scizor+Rotom-W, were commonly switched in vs my Infernape. This meant getting a free U-Turn off and then - you guessed it - going to Weavile again, and then yet another 50/50 ensues.
Although the problem with VoltTurn itself isn't Weavile, like pif mentioned, Weavile definitely helps apply incredible pressure when paired with this core. The Latias example applies again: if you switch in your Latias vs a Volt from Rotom-W, and they play Scizor, then they have to fear HP Fire. However, with Weavile present, this ultimately makes getting rid of Latias (or whatever other mon you use to counter VoltTurn) that much easier while keeping your Scizor healthy in the back.
To briefly touch on the Weavile needing support part, I don't think this is bad at all. Every mon needs proper adequate support to function properly. Since I'm most acquainted with OU, I'll give Magearna as an example. Its sweeping capabilities are unparalled, as it can run SG + 3 atks or even double dance, and its movepool has expanded to more than Z-Fight + boltbeam, as even Z-Fairy and Z-Steel have seen decent usage to blast through would-be counters. This usually benefits the Magearna user, as if you're running say, dual stab, you know you want to build around getting rid of mons like Heatran. If your opponent stays in with Heatran having already seen Flash Cannon, they might proceed to get blown away by Z-Fight. The opponent is left on the back foot, trying to scout for Magearna's set instead of staying in with Heatran to not let their mons take too much damage beforehand. Now, I don't think that Weavile in itself is comparable to Magearna, but I do think they impose the same mentality on your opponent - one wrong prediction, and it opens up potential for not only Weavile itself, but the other mons on the team (like I mentioned before, trapping Latias or knocking off Rotom usually meant that my scarf Nape had a lot of fun). If anything, supporting Weavile isn't hard at all, since the aforementioned Scizor+Rotom-W are very easy to throw on a team due to their amazing synergy.
Interestingly enough, a situation arose in a match with
teal6 on the ladder yesterday. We were both using standard VoltTurn teams until a turn (turn 3) came down to my Scizor vs his Gardevoir. Seeing as my team lacked a Gardevoir switch-in, the 50/50 came down to whether I would BP early on to pick Garde off, or U-Turn on the switch and bring in my Weavile to Knock something off. I proceeded to get both the 50/50 with Scizor and the following 50/50 with Weavile afterwards wrong, which meant an automatic loss for me. Is that really fun to play? A match shouldn't come down to two 50/50's in the first 4 turns of the battle. It wasn't exactly fun for me to jump into a tier for the first time governed with such a bland but dominant strategy like this for about 90% of games played.
Overall, I'm definitely going to have to agree that Weavile is unhealthy for the tier. I think that M-Garde and M-Lati are stupidly powerful and over-centralizing as well, as others before me have mentioned. I didn't really struggle as much with M-Garde, mostly because of 50/50's with my Weavile or Scizor that resulted in my opponent staying in on a CB Crash or BP. M-Lati, however, finds it increasingly easy to set up and at +2, anything that doesn't resist either SP or your coverage move (or with Toxic/T-Wave) is basically an automatic loss. People usually rely on steels as their answer to this, but it was extremely easy to just switch out Surf for HP Fire and destroy every Scizor that thought they could come in for free. I do think that this tier has potential, and banning Weavile is a step in the right direction.