If multiclassing works the way it did in the last play test, then a Wizard 19/Cleric 1 can use high-level spell slots to cast heightened cure wounds and be a very effective healer. I suspect this is why mass cure wounds is a a separate spell and not just a heightened cure wounds.
That's not a game-breaking exploit, but it is a little bit of niche erosion that may piss off another player who wants to be the party's dedicated healer.
Because proficiencies are so easy to come by, a Wizard 19/Cleric 1 can also pick up Perception, Investigation, Stealth, and Arcana, and thieves' tools proficiency. With the right feats and racial traits, and a decent Con, you can have a decent AC and HP. At that point you are like a 1-man adventuring party with nearly full wizard spellcasting. In theory you could be as tough as a non-optimized fighter.