Phoenix was in meta in patch 7.31, but in the last patch, Fire Spirits were buffed. As a result, the win rate increased from 48.48% to 50.10%(on the day of the patch's release), but now it has a 47.98% win rate and a 3.99% pick rate. Even though Fire Spirits only got +5 damage at all levels, this is a noticeable effect on enemies in the early game. In addition, Phoenix got +1 hp regen point gave him a little better durability at the beginning of the game.
However, even after the improvement of Fire Spirits, the best thing in Phoenix is his Aghanim's Shard. Sun Ray becomes castable during Supernova, and casting Supernova during Sun Ray no longer ends and slows affected enemies by 12%. Sun Ray is a powerful ability with 14/20/26/32 base damage per second and 7/10/13/16 base heal per second. Furthermore, all this damage\heal plus a percentage of their health. The percentage increases as the beam continue to fire.
Then firstly maxed Sun Ray, Ultimate with Icarus Dive.
Veil of Discord to increase magic damage, Tranquil Boots, because all Phoenix skills cost a percentage of current health, Magic Wand, which upgrades to Holy Locket or Spirit Vessel, in case you need to reduce the regeneration of enemies (like Bristleback, Axe, Centaur Warrunner and similar) and Aghanim's Shard.
In the late game, players usually buy Aghanim's Scepter and Refresher Orb, if the game continues to mega late, you need to look at the situation. The team lacks control – purchase Scythe of Vyse, the enemy team has a lot of control and target abilities – buy Lotus Orb, enemies run away quickly – make Shiva's Guard. Likewise, item build through Octarine Core, Radiance, Heart of Tarrasque, Heaven's Halberd, Kaya, and Sange is also excellent.
Phoenix is best paired with such heroes as Mars, Faceless Void, Disruptor, and Enigma, with all heroes who can fence off Supernova from the enemy team and keep them in a radius (1200) of the effect of the ultimate.
In summarizing, some of the upsides include that Phoenix has massive magical damage and healing, abilities to act on a large radius, owns many debuffs, and good escape options if you press the buttons correctly. The most significant upside of it all is the absence of accurate and reliable disables, health costs along with mana costs on most abilities and low armor, the ability to maintain a good position, to aim, and almost no spell immunity piercing effects.
In general, Phoniex is good support and initiator, but only in capable hands, which proves his winning rating, so there is no need to pick him if you do not have unique skills to play this hero.