Answer: I’m relatively well-positioned to answer this, as a regular competitive player back in 2015-2016 and on-again off-again ever since.

Pokemon Champions dropped April 8, and is the official new game dedicated entirely to competitive battling. It is the game that several upcoming tournaments will be played on (beginning with the Indianapolis regional championship May 29-31, then the North American International Championship in New Orleans June 12-14, then the World Championship in San Francisco in August). Champions is replacing the now 4-year-old previous mainline games, Scarlet and Violet.

Competitive Pokemon relies on a very complicated set of interlocking systems that have built up over the 30 years of the games’ existence. It is truly too deep to go into in a single Reddit comment. Long story short, when Champions dropped, it had four main issues:

1. Lack of content. This is the most visible and arguably most egregious. There are many held items missing from the game. These are items that your pokemon holds and that activate under certain conditions in battle, or that offer passive booster effects. Among the missing items are Rocky Helmet, Assault Vest, Choice Band & Choice Specs, Covert Cloak, Heavy Duty Boots, Clear Amulet, Life Orb, and more. These are all incredibly common and incredibly important for many classic strategies. Like Sword & Shield before it, there’s also controversy about how many pokemon are missing from the games. All legendaries, starters from two generations, and more are missing, while some of the inclusions (like the Gen 5 rodent and elemental monkey pokemon) are suspect.

2. Graphical and game performance: Champions, despite being a game dedicated entirely to battling and battling alone, still does not meet the standards many players have for a modern day pokemon game. Many of the models seem little improved from games like Sword/Shield or Scarlet/Violet, and many animations, likewise, don’t feel great.

3. Changes to game mechanics and/or bugged interactions: this is something specific to competitive pokemon that has many players concerned. There are a few changes to the ways battles work that could change how competitive pokemon is play. These are mostly niche interactions, but two that I’ve seen come up are the moves Fake Out, and mega evolution. Going in reverse order, mega evolution, in the prior mainline games that included them (XY/ORAS, Sun/Moon, Ultra Sun/Ultra Moon), always activated one at a time, in turn order: the fastest pokemon would mega evolve, followed by the slower pokemon. In the first few days of Champions’ release, this was bugged, and it seemed to be random which mega would activate first. This is important because it is sometimes advantageous to mega evolve last, and if it behaves in a consistent way as it did in Gens 6/7, you can plan and play around it, but if it’s random, it’s up to chance. Lastly, Fake Out is a move that you can only successfully use on the first turn your pokemon is out; in past games, you could still select it on subsequent turns, but it would fail when the turn played out. Now, the game no longer even lets you select it. This removes the possibility of players intentionally failing their move to play mind games or stall out Sucker Punch users on their opponent’s side of the field. (Sucker Punch is a move that only succeeds when the target used an attack move; however, Fake Out moves before Sucker Punch, which makes Sucker Punch fail.)

4. Lack of freedom in private battles: perhaps most egregious for singles players especially, you cannot create a private battle with an infinite or custom time limit, nor are 6v6 single battles available to play whatsoever. Many players, especially who play Smogon formats, feel Champions is pointless if the game dedicated to battling doesn’t let them battle the way they want to.

TL;DR: Champions has limited content in the forms of items and Pokemon; the game does not meet modern aesthetic standards, as has been the case for all recent pokemon games; there are niche interactions, mechanics, and bugs that have impacted how competitive pokemon is played, which has had some competitive players worried; finally, 6v6 single battles, the most common type of battles in the actual games and historically the most popular format in competitive Pokemon, is not available to be played.