But since multiple sources don't stack, it stays balanced.
Reading through the playtest documents, I'm finding little +/- 1s and 2s tucked here and there, like mob rule for Kobolds and Rats in the Bestiary, or -1 to attack when prone. It kind of defeats the purpose of advantage/disadvantage simplifying everything to mix the static modifiers back in, and static mods mixed with advantage/disadvantage I would even consider more complex than static modifiers alone.
The problem with using advantage and disadvantage exclusively is eventually there will be so many sources of both that someone will almost always have both advantage and disadvantage. That means any additional sources of advantage/disadvantage become useless.
If critical hits remain as they are, not being able to critical is a nice minor penalty (for prone perhaps)
and perhaps we could offer criticals on 19 or 20 (for bless perhaps).
Rather than apply disadvantage, you could not be able to benefit from advantage (and vice-versa - not being subject to disadvantage seems like a good benefit for a mob).
For group effects, to saves perhaps, you could use the best or worst save bonus from the group rather than just a numerical effect.
For the healer speciality, you could step up HD rather than maximise them.
Overall, I don't mind the odd bonus, but they should *never* stack - the highest magical bonus to any given dice roll or defence should apply.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.