Both are pretty flawed and fairly niche roles. They add an annoying element of micromanagement, while not really being that much more useful than a vanilla unit would have been in that slot. It is true that disabling a healer tends to lead to disaster more often than disabling an attacker, since you'll have less healers on the map compared to attackers, so disabling one might be critical.
Honestly - think of a situation where you'll want to switch a healer to an attacker. How often does that come up? Almost never, right? Now, how about switching an attacker to a healer? Pretty easy - maps tend to swarm you early game then drop huge bosses which need to be tanked later.
Generally, the best multi-role units are ones who toggle with a low CD skill like Shiho, or ones who simply do both at the same time like Eterna.