So, it's broken because it's a good revenge killer? First of all, for the umpteenthtime, Specs or Band ruins Deoxys. Life Orb is infinitely better.
I've already pointed out, if Deoxys doesn't carry shadow ball, it can't even take on an Azelf with a -Sp. Def Nature 1 on 1. If Deoxys can't score a SE hit, it will have a hard time taking down anything in one hit, and thanks to its pitiful hp means it won't be taking many hits.
Mixape can OHKO almost every OU Wall in the game and the only thing that Deoxys beats it in is speed. Mixape has WAY better typing(psychic is one of the worst types for taking hits and attacking) and a MUCH better movepool.
I'm just going to list pokes that can deal with Deoxys pretty well.
Blissey - Doesn't like Psycho Boost followed by Super Power.
Cresselia - Fine, Shadow Ball does less than I expected.
Bronzong - Fine, but it still doesn't like HP Fire.
Metagross - Fine, it counters Deoxys A.
Heracross - ScarfCross is still slower, dies to any Psychic move.
Jirachi - Fine, although HP Fire makes it less of a counter.
Forretress - HP Fire says die. Damage: 46.89% - 55.08%, that's from Life Orb 252 SAtk Timid using Thunderbolt
Scizor - HP Fire says die. Otherwise, a relatively solid counter.
Rhyperior(especially in sand) - Life Orb Grass Knot from 252 SAtk Timid does 96% min to 252/252 Rhyperior in Sand, Damage: 54.61% - 64.29% is the same Deoxys using Ice Beam on Max HP Rhyperior (much more common)
Scarfgar - Dies to any Psychic attack it tries to switch in on
Raikou - Damage: 63.02% - 74.22% from Life Orb, Timid, 252 SAtk Psycho Boost on 252 HP Raikou, which no one really uses
Lucario(especially with the Swords Dance + FS + Extreme Speed) - Counters do not need Focus Sash, cannot switch in on Superpower
Arcanine - Damage: 58.51% - 68.46% that's with Adamant Nature, 252 ATK EV's, Choice Band using Extreme Speed, not a counter and can barely switch in.
This is just off the top of my head. Granted some of these depend on the set, I based it off my ideal set: BoltBeam / Superpower / Calm Mind.