My experience in ranked games and scrims with Io so far has been pleasant. I'll share my findings here:
Luna is really good early game no matter which queue you're in. In ranked games, it is especially easy to take advantage of early game by putting her on/next to point behind cover for the main tank. This will enable your team to get free cap time and create early diversion for effective flank plays should the enemy decide to focus down Luna or the main tank, whoever is on the point at the time. Keep in mind that if the enemy team is starting off with bulldozer as first item, this is to your advantage because they're throwing away early & mid-game as your heals become more effective with the absence of cauterize 1 & 2.
The 2nd round and the 2nd push phase is when enemy will start picking up bulldozer. Once one person on the enemy team has bulldozer 2 (that isn't a tank), your best bet is putting Luna behind cover NEAR the point, that way your main tank is able to effectively heal up after disengaging the point. This is going to allow you to exclusively use Moonlight on your offlane, where a majority of your team is going to be playing, and where teamfights are going to occur.
Protectors looks like a good card on paper, but in reality it's pretty bad. This card forces Luna to play next to you, and if you're using it like a defensive grohk totem, you're not getting much done. Sure, your offlane is able to sustain better early game, but that only extends to the first round, where neither team has ultimates up. If you're playing on a map like Serpent Beach, and you're running Protectors. The chances are that you are going to misposition yourself to heal the main tank, making you a really easy target to push into or flanked. (For reference you want to stay on the high ground as a support on Serpent Beach. Playing next to Sundial or in the L leeway next to sewers will inevitably get you killed should a flank or offtank push into you from either side, since your escape route is almost non-existant, and your team is cut off from all heals when you're playing for life)
Io is especially good if you're drafting for triple DPS composition. Her extra heals and utility is comparable to Pip, and your team is able to 5-man zone due to Luna being able to capture objectives. Her worst match up is against Willo, as her deadzone and general damage output on top of her ultimate makes it easy to overload her healing potential. She has similar problem to the old Celestial Touch Jenos, yes you can heal 3 people at the same time, but your whole team is vulnerable to burn strategy. This is especially apparent once the enemy team has caut 3. You can also draft Io as an offhealer, running Goddess Blessing to pocket hard-divers and those who's disengaging from the team fight.
She is a good champion to pick into a flank, since she's able to recall Luna for the stun and extra damage. If you're able to land 3 arrows and recall Luna next to you when a flank does flank you, you win the trade and get a free kill 90% of the time, because Luna stun lets you land finishing blows easily. Talus is the exception because of his DR and lifesteal cards on top of the high consistent damage, you will be forced to use your Lunar Leap or ultimate to stay alive should he flank you.
When transitioning to late game, you really want to stray away from putting Luna on the objective right away because of late game bulldozer. This is when you want to put Luna in a safe location close to the point, so your main tank knows where to back up into safely to get heals. Luna heal range is pretty forgiving too, so this isn't the hardest thing to do on most maps. Because a lot of chaos will be happening towards the last couple of rounds (ultimates up, cauterize 3, quick switches between hyper aggression and ego ults), you really really really really really need to make sure you're running Broken Diety and Spirit Arrows. I cannot stress this enough. You have to be able to move Luna again and again at the moment's notice because the flow of battle changes dramatically and incredibly fast in high MMR games. If you're running neither of them, you're signing up to lose late game.
One of the common comments I see is about Moonlight Garden vs Sanctum of Faith. The reason why you don't want to run Moonlight Garden past 3 is because supports aren't meant to hardpeek/egopeek for damage. The only exception is if you're playing offhealer Io (not what this build is about to begin with), then Moonlight Garden is favored over Sanctum of Faith. You don't play Io the way you do with Mal'Damba, Seris, Ying, or Furia. If you ever get forced out of LOS to heal because you're low on health or you ran out of Moonlight charge, you're going to inevitably fuck someone on your team over because of multiple scenarios:
- Recalling Luna denies your main tank of healing, making it easy for the enemy team to pressure him out/get a free pick
- Hiding out of LOS means your team can't play aggressively without playing off of ultimates. If they get bursted or take more than adequate amount of damage, they're most likely to get picked than not within the next 2 seconds
- IF you're not running Restored Faith, you're forced to out of combat heal to get your health back, and forces your team to play defensive/reactive. Very identifiable opening for the enemy team to switch into aggression and open/finish fights with ults
You can avoid all these scenarios if you ran maxed Sanctum of Faith with Restored Faith and some points into Moonlight Garden, as you're able to sustain yourself better WHILE outputting effective heals. I try to compare these three cards to Jenos running Retrograde, Astral Cycle, and Relativity. Why would you give up more healing for the sake of more potential damage? You already have 2 DPS and an offtank to deal the damage, there's no reason for you to be playing like a DPS instead of healing your teammates when you're playing the support role. It's just fundamentally wrong, and I discourage playing supports like off-healers when you're the only healer on the team, as you become insanely easy to punish in high MMR games, especially in scrims.
Hopefully this is able to clear up some confusions and provide insight into discussions in the comments section.