I have no problem at all main healing as SMN. With earthen ward, phalanxga, aeriel armor, I can reduce damage main tanks take, remove all the damage backup tanks and party members take. Nix AE damage and allow myself to rest for longer periods of time allowing clear mind abilities to stack. Then when my party is taking low damage I can shift the focus to more offensive buffs like Fenrir's stat buffs. Put the eva/acc buff on with a ninja tank and it buffs their evasion so signifigantly it saves me MP easily while increasing damage output by buffing all the melee's accuracy.
Furthermore those buffs reducing damage makes SMN a very hate efficient healer, as instead of just directly curing the damage like a WHM they don't take hate for that damage reduced, as well as the tank retains more hate by not bleeding it off.
People who can't main heal as SMN are either people who are in a party that was in some way broken to begin with and a WHM would have failed, or they don't take advantage of all their abilities.
Furthermore those buffs reducing damage makes SMN a very hate efficient healer, as instead of just directly curing the damage like a WHM they don't take hate for that damage reduced, as well as the tank retains more hate by not bleeding it off.
People who can't main heal as SMN are either people who are in a party that was in some way broken to begin with and a WHM would have failed, or they don't take advantage of all their abilities.