I've been experimenting with Mercy in sandbox mode, trying to find some practical application for it, and am coming up short. Like Janus, the DoT means it is terrible against shields and requires 3 passes to kill a T2 Pgen. Also like Janus, it's great at destroying groups of T1, T2, and Engies.
Unlike Janus however, a single Flak can kill the entire squad of Mercy before they hit. A Janus will survive at least 2 passes over a Flak, dealing a lot of damage each time. Coupled with not being reusable, I'm just not seeing any use for the unit. The change to DoT means it can't even be used as a T4 deterrent anymore. So what's the point of Mercy these days?