roaming mend is like a microcosm of what makes sorc healers the best in PVP right now, which is that it's just so much easier to put up big numbers
kolto probes dropping is inevitable even for the best op healers. you get stunned, mezzed, line of sighted, and sometimes just overwhelmed. and it costs you 2 gcds every time this happens. meanwhile sorc has NO equivalent mechanic to worry about denting their HPS
without sufficient clump, operative healers suffer significantly. you get far fewer hps and TAs from nanotech and kolto waves goes from being OK to straight garbage. sorc puddles are big, have no requirement that you sit in them, and just aren't necessary anyway. And the range on roaming mend is so big that clump is completely unnecessary.
op healers have significantly less of the burst you need so badly in PVP. everything that is instant is weak (minus the instant injection you get, at most, every two minutes with stim boost) and everything that isn't is a cast that you may never actually get off. sorcs on the other hand have *multiple* options to save someone who gets low. I love the instant injection/infusion with stim boost but it just isn't enough. If operatives could do more HPS overall this lack of burst would make sense. But sorcs beat ops in overall hps AND burst.
sorcs also have the upper hand in survivability cooldowns. cloaking screen is less reliable than both barrier and phase walk due to not being available while stunned and the possibility of being immediately knocked out of stealth. exfiltrate, shield probe, and evasion are all very useful but each has major drawbacks: each use of exfiltrate is a wasted gcd from an hps standpoint, shield probe just doesn't absorb all that much damage, and the usefulness of evasion varies wildly depending on how well you time it and what spec is attacking you. it is particularly meh against the AP PTs dominating PVP at the moment, being completely useless against their two scariest attacks: thermal det and energy burst
p.s. I don't even know where to begin with merc healers