I'm wondering if Phoenix is another one of these cases where the Dota 1 community input is just off-base, like how Ember Spirit was supposedly super broken and Earth Spirit merely ok. Why would I ever pick this guy for support? He's super slow, super squishy early game, needs levels, has fairly long cooldowns given the strength of his abilities, (presumably to synergize with the benefits of his ulti?) has no powerful early burst dmg, (all of his dmg is dot based) and cannot even disable except in the circumstance that his super long setup ulti is allowed to go off around enemies. Sunray appears to be a useless skill in its current incarnation and dive is easily punished with the latest patch changes. He clearly wants some farm- mek seems almost core- but as a support he won't get a mek in the relevant phase of the game for it and as a mid, he'll be shutdown hard with his low base dmg, lack of instant wave push, cruddy move speed, and 0 armor. The only things he really has going for him is range on his dive and the ability to screw up last hits with properly timed spirits.
I really like the concept and how he looks, but I think he's going to need either buffs to undo the damage or a rework entirely. I also freely admit I'm a pretty middle of the road player, by mmr at least, so I'm sure I could be missing things and/or just playing him incorrectly. Would love to be proven wrong, but I think this is another case where the competitive scene and metrics in Dota 2 are going to say something entirely different from the Dota 1 crowd.