Since I'm bad and need Pheromosa to win, I've played this mon more than anything besides Aegislash so far and can tell you that the only similarity between this and Deoxys is that they're both fast and vulnerable to priority. They play very differently. Deoxys is something you get in after a slow U-turn or sac and start trying to wallbreak with howitzers like Psycho Boost and it's excellent set of coverage moves with tons of potential for super effective damage, while keeping itself protected with Extremespeed. Pheromosa doesn't have a tool like Extremespeed nor does it have to raw power to just brute force it's way through walls. What it has going for it is an extremely predictable but flexible moveset that allows it to hit most of the tier for solid damage. It has extreme difficulty against the fat teams out there with Aegislash, Toxapex, Mantine and the like switching in all day every day. This should instead be used as a trump card against offensive teams, as they tend to lack recovery and can easily see most of its members worn down to, say, the 60% range, at which point Pheromosa can pretty much grab a kill against them any time it comes in safely and can really only be answered by revenge killing. The main advantage it has over Deoxys is a STAB U-turn that with it's blistering speed, can get it out of harms way while accounting for the opponent's potential double switches, making it one of the most frustrating pokemon to lock down and put away for good. Just because you have Aegislash doesn't mean your team isn't weak to this thing since it'll just U-turn into a check as you switch in the shield, losing you momentum.
On a side note, doubt it'll see much of a niche in Ubers. It just doesn't have the raw power to get through your defensive stalwarts that plague the Ubers tier.