I'll be brutally honest. And this is coming from someone whose favorite Alola Pokémon is Decidueye.
The only niche I see Decidueye having is as a special pivot-slash-defogger-slash-spinblocker, with Defog + Spirit Shackle + Roost being basically it. Thing is very few teams ever need something like that, so, yes, it's pretty decent role compression, but, if you don't need all of those (and being a defogger plus a spinblocker that cannot setup hazards already is kind of... awkward, to say the least), you're probably better using a different Pokémon in your team. Baton Pass sets are outclassed by Celebi, too slow to be a sweeper, too frail to be a wall.
That said, it's still a cool Pokémon, with a cool design, but that it desperately wishes it had better focused stats, be it more Attack, more Speed, or more bulk.
Or, a better movepool. It really feels like it needs certain moves to fit better in certain roles (-cough-Taunt-cough-Jump Kick-cough-). Right now, it's stuck at being a low jack-of-all-trades, master-of-none Pokémon, and an awkward one at that.
Decent Attack, but low Speed means you're outclassed as an SD sweeper, low bulk too means being a bulky sweeper is probably out of your reach (hehe, get it?), that same low bulk means you're eh-ish as a defensive Pokémon, and a limited (yes, limited) support movepool means you've got the short end of the stick there too. Hell, I expect to get Tailwind from move tutors, and possibly a few other toys for both support and offense (Long Reach Knock Off sounds interesting)... if we do get move tutors. Or maybe even better special options for a mixed attacker! And then your 4MSS pops up lol.
Also, Assault Vest is a waste of your movepool. Namely, your access to both Defog and Roost. Dhelmise probably runs a better AV, with its higher bulk, and higher attack, and lack of reliable recovery.
That said, to point out useful info, good partners for Decidueye are trappers, the most common ones, Dugtrio and Magnezone, sticking out. Call out a switch-in correctly (especially on something like a Heatran, a Celesteela, or others that want to come in to wall you), Shackle it in, switch to your trapper of choice, and you can get a pretty easy KO. Sadly, this does mean you're even bigger Pursuit bait, and your low physical bulk means you're probably dead anyway, even if you try to avoid the extra damage by U-turning out.
Honestly, the best I could visualise Decidueye being, is a support-slash-special pivot with offensive presence, which means you're probably going to want to go for Attack and Special bulk. Still trying to find an EV spread that could optimize this niche.