Kyurem is overcentralizing.
Kyurem is a 6-0 machine against unprepared teams.
Kyurem alone renders queueing up Mega Swampert Rain tantamount to wintrading.
Kyurem makes constructing traditional balance teams an absolute nightmare.
I don't think Kyurem is broken. It is not the only reason balance feels impossible to play, and as a unit that relies on its uninvested natural bulk to function, it itself suffers from many of the same issues that plague balance as a whole. Offense has been king in PRO for quite some time now, and I think there is a different discussion that is worth having about the metagame at large.
It is functionally impossible to reliably remove Stealth Rock, unless you know your opponent's sets either because you've played them before, your guild has, or your friends have.
Let's take a look at the state of our hazard removal. The only viable Rapid Spin user is Excadrill. Defensive Excadrill sets have been pushed to the wayside by offensive powercreep since about 2016, and offensive ones may remove Stealth Rock once or twice, but that's far from its main role and it is far from good at it - Excadrill certainly does not scare out Garchomp or Landorus-T. Tornadus-T is weak to Stealth Rock, and currently generally runs offensive EVs to make its Supersonic Skystrike hit harder. It's one of our better options, but is liable to being worn down over time (by definition, removing Stealth Rock with Tornadus-T means it is at 75%), and is not living Stone Edges or Z-nukes from any of our offensive hazard setters. Utility Serperior works in a pinch, but has no recovery bar the unreliable Synthesis, which it can never fit on a set. Zapdos is weak to Stealth Rock itself and is generally tasked with checking Mega Lopunny - something it cannot do reliably if it enters the field at 75% HP. It too is liable to getting nuked by offensive Z-SR sets. Landorus-T has longevity issues, as it is frequently used as a blanket check, and more importantly usually belongs on the sort of team that would rather trade Stealth Rock with its opponent than remove them from both sides. Gliscor is quite good at removing Stealth Rock - however, Defog Gliscor sets often do little else besides removing items or checking Heatran. Rotom-Wash is a traditional balance staple - but also a loss condition in the face of Substitute Kyurem. Its bulk also does not hold up nearly as well in a world with Z-moves, but also no Toxapex to take advantage of to pivot freely. Instead, Slowking takes pitiful damage from uninvested Volt Switch and seizes momentum itself with its slower Chilly Reception. Defog Skarmory is a niche option which deals with many of the physical Stealth Rock setters well, but virtually commits you to playing Stall. Furthermore, all Defoggers must deal with the specter of Defiant Eviolite Bisharp and to a lesser extent Annihilape, which makes the choice to remove Stealth Rock often a trade of a long-term annoyance for an immediate short-term crisis. In contrast, we have a wide array of Pokemon that are very good at keeping Stealth Rock up against virtually anything - Z or Taunt Landorus-T, Z or bulky Garchomp, MG Clefable, Mega Diancie, Mold Breaker Excadrill, Offensive Tyranitar, Z Heatran, Z Kommo-o...
Why is this relevant?
Both Substitute and Specs Kyurem are Pokemon that thrive off coming in on the double switch or a safe pivot move from a teammate, such as Mega Scizor's U-Turn, Slowbro's Teleport, or Rotom-Wash's Volt Switch. These repeated free switches with very limited safe answers and the risk of Freeze from primary STAB are the heart of Kyurem's oppressive gameplay cycle. However, there are no free switches when Stealth Rock is on the field. Losing a quarter of its HP renders Kyurem in KO range of many strong and fast attackers and greatly limits the amount of times it can afford to fish for freeze. It also limits the amount of PP Substitute Kyurem can remove from an erstwhile check before being forced into uncomfortable mindgames. When Kyurem is allowed to play its game unmolested, it is absolutely worthy of a ban - however, I would argue that no competent player should be allowing this to happen on anything resembling a consistent basis.
Another aspect I think is sometimes underappreciated about Kyurem's presence in PRO is that Kyurem-Black's legality actually makes regular Kyurem significantly easier to play against. Z- and boosting item sets tend to be the sole provenance of Kyurem-Black, making it relatively easy to assume that the unfused Kyurem you are facing in team preview is in fact SubRoost or Choiced.
Finally, the archetypes that Kyurem destroys were not very good to begin with. With or without Kyurem, Rain has a disaster matchup against Chilly Reception Slowking, Aurora Veil, and many variants of offense. With or without Kyurem, Balance is not going to recover unless a litany of other gigantic offensive threats are removed and hazard removal achieves some sort of relative parity with hazard setting (or a certain item is released). As for Stall, well, it deals with SubRoost Kyurem surprisingly well. Calm CM Unaware Clefable was already forced for reliable counterplay to threats like Z-Bug Volcarona, Manaphy, and LO MG Clefable. With Wish and Heal Bell support, it has no problem sitting in front of standard SubRoost Kyurem forever - even when it gets frozen. Aurora Veil, on paper, is simply favored - Ninetales-Alola is one of Kyurem's better checks, and Kyurem should never really be able to come in more than once or twice. Other offensive teams are similar in this regard.
I am not going to lose any sleep over Kyurem potentially getting banned. However, from my perspective, it warrants tiering action about as much as Volcarona - that is to say, perhaps in theory, but not at all in practice.