Zens ult is a slow charging ult that should not be relied to heal the team alot of the time and you cant rely on it to quickly heal up other teammates as you wont even have it as much.
His damage orb is his utility other than well his ult but again slow charging ult.
Zen mercy is horrible unless you all play on collapsing on the enemy quickly which is very unlikely outside high end ladder.
Why is it horrible? Well mercys utility comes from rez and damage boost but with zen heals it means mercy has to heal alot more than she should have to ideally so you can forget those 2 or risk just toppling over. Mercy barely has more heals than zen sure her ult comes fast but youre relying on your support ults to get any real amount of heals.
Zen needs a bap or an ana really...atleast a moira...brig is much better than mercy atleast lol