The other reason Synastry isn't OP is simply because, in order to use it to its maximum potential without seriously overhealing like crazy, you need to be in a situation where a tank will be taking damage for an extended period of time while you'll also be tossing direct heals onto other players as well as it doesn't work for AoE healing, and if you're in a situation where EVERYONE is taking damage, you should probably be AoE healing instead of single target.

That's fairly specific in its nature. Also, you may cast Synastry on the tank only to realize moments later that the tank is taking too much damage to be saved by Synastry alone, meaning the ability will be wasted.

I'd rather Synastry had an effect like "25% of all healing done to the target is also done to a 15y radius around the target", giving us a unique but clutch AoE option. Or maybe they could make it a smart heal that healed the nearest, most injured player for 50% of the healing done to the target.

Right now, Synastry just strikes me as an ability best used when the tank is taking normal damage and some idiot DPS is also getting themselves killed, like saying, "Here, tank, have this. I need to go save this moron..."