Sun is a very strong team style between drought Vulpix making it convienient to setup and Chloro abusers hitting a hard to outspeed speed tier of 28 or 30 (Bulbasaur only). The main three ways to counter it is (1) priority spam, (2) webs with 19-20 speed teammates, and (3) opposing weather. Additionally, scarf Vulpix can be a big threat in its own right too.

Priority is very good in BDSP LC so it's the easiest to justify on most teams, but it won't fit nicely on all and debatably you need something like fake out + other priority to actually break through chloro mons.

 Webs is certainly good, but with only Surskit and Spinarak as setters, it can sometimes be very hard to fit onto teams and most web teams aren't necessarily going to pick a 19/20 speeder over, for example, Gastly which helps with opposing webs. All that said, this will only work for the 13-14 speed chloro abusers. Bulbasaur will outspeed 19 speed mon even with webs and will speedtie diglett/elekid (which wouldn't help much with Bulbasaur anyhow).

Opposing weather is not even close to as good as sun. The only auto setters are snover and hippo, both of which are weak to the fire/grass core of a sun team. Additionally, water is a very common type making hippo/shrew sand more of a liability than a benefit most of the time. Manual rain is always going to be outpaced in tempo compared to the auto sun Vulpix brings.

I think sun ultiamtely has elements of brokenness and uncompetitiveness. The former due to the very limited options to stop sun sweeps and the latter due to it being a huge matchup fish.