As a player who loves to play and create stall teams, I am naturally drawn toward the most common tiers of OU and National Dex, which is also where stall tends to be at its strongest. Generally stall is most common in National Dex, where the existence of Mega Sableye opens up the door to a very effective, (and somewhat annoying), form of hazard and status denying stall. The most notable effect of this is it allows the stall player to use Chansey over its more common OU evolution Blissey. The reason for this is simple. Chansey is able to use eviolite which makes it bulkier than Blissey, but because it lacks Heavy Duty Boots it is very easily worn down over the course of a long game. Mega Sableye can control hazards very effectively which allows Chansey to earn its place in National Dex stall.
The only issue however is that National Dex is much stronger, with megas and Z-moves pushing pokemon like Mawile or Kartana to new levels of strength. Because of the significantly higher power level there are many more threats to stall, most commonly, set up sweepers. In OU most generic setup sweepers can be checked by the likes of Corviknight, Haze Toxapex, or even Whirlwind Hippowdon. In Nat Dex though, there are Pokémon like Mega Charizard X, Z-Garchomp, and sometimes even a stray Calm Mind Z Tapu Lele. This means that many standard stall teams in Nat Dex are forced into running an Unaware Pokemon such as Quagsire or Clefable. At first this doesn’t seem like an issue, but it can create a large gap in teams.
Pokémon with Unaware (mostly Quagsire and Clefable) generally only serve as a check to sweepers and they are outclassed by other choices with similar typings/niches. For example, Quagsire is very good at walling the aforementioned Mega Charizard X, but it is near useless when it comes to being a general ground type wall. Something like Hippowdon is generally a safer pick for it’s better bulk, utility, (i.e. stealth rocks, whirlwind, etc), and a better typing. It’s also outclassed by Toxapex as a water type wall for many of the same reasons. Effectively it is wasting a slot, as it is only checking very specific Pokemon, and it performs way worse than other choices. Also, a main factor of stall is having a strong/consistent win condition. This is a Pokemon or strategy used as the primary way of KOing opposing Pokemon. Generally this is a strong wallbreaker/sweeper like Garchomp or even Kyurem, a pursuit trapper such as Weavile or Tyranitar, or in specific team compositions, more bulky Pokemon like Calm Mind Clefable or sometimes Swords Dance Gliscor. The problem here is that the added slot of an Unaware user takes that spot from that offensive presence in a lot of stall teams. Also using a Pokemon with Unaware means that generally the defensive core of a team will be more easily muscled through, and thus it makes the team a lot less flexible, and there will almost always be a specific Pokemon or type that your team will struggle to handle, (some common ones are Specs Tapu Lele, Heatran, Gliscor, Tapu Fini, etc).
So in reality, I think currently stall is best in OU because it can be built without the restriction of needing an Unaware user. Although it lacks the ability to use Mega Sableye, it also doesn’t have to worry about Z-moves and other megas. Because of this it can more easily fit everything it needs to wall as much of the metagame as possible.
So overall National Dex Stall is still effective ( and certainly fun) but requires a very strict set of Pokemon. It’s important to know the metagame very well whenever you attempt to build a stall team, and with Nat Dex it’s very important to make sure that you cover as many matchups and strategies as possible. If your looking to build or play stall for the first time OU stall is much easier to construct and play, and can be much more flexible and less demanding as a player. Good luck! And have fun stalling!! :)