Right now, it only has 2 good users: Anubisath Idol and Enchanted Broom, and a lot of people don't have/use a flawless stone on Enchanted Broom. Silithid Hatchling is around too but lacks a good move in the 2nd slot, substantially reducing its value on a team.
All other current users of Sandstorm are either Elementals who don't benefit from it and/or have a movepool reliant on multi-hit or AOE attacks which Sandstorm would itself nerf, or both (Tiny Twister).
In a little over a week, Qiraji Guardling will become available, adding another viable user, one which can have up to 325 Speed to launch a 2-turn stun against most pets, effectively giving it a free setup turn as well.
Fluxfire was annoying, you had to play around it, but that was as simple as using a strong elemental or useful magic type. Reflection was overpowered but had a long cooldown. Both could be played around without much effort...
...Sandstorm wrecks entire segments of the pet battle population single-handedly, and cannot be played around easily. If a team has 2 Sandstorm users, with its short 3-turn cooldown, you're not winning most weather wars. Pets that rely on multi-hit attacks all become useless, as well as all DOT-based pets.
It's not unbeatable (then again Fluxfire and Reflection weren't either, not even close, 1 Oily Slimeling was enough for me to win over 90% of matches against Fluxfires, and Reflection can be predicted), but it is extremely disruptive to the metagame. I've played against it off teams with JUST the 1 Anubisath, I've also taken to testing it myself with Anubisath + Broom. It IS overpowered and it WILL disrupt the metagame heavily. I'm winning matches against all kinds of things without even losing a pet to a KO now.
I would strongly advise one of Blizzard's next hotfixes to address this issue, rather than waiting until 5.3.