1) This cannot stack with regular damage. The opponent must be fully healed for this to damage them. This is a vital distinction to consider.
2) This costs more than Poison. (not much more but more 1
~= 3
, 5
~= 2
)
3) People can defend against this with PC, Denial and Max Hp gain. People can defend against Poison with Purify, Healing and Denial.
4) Every element having access is not a balance issue unless there is a problematic synergy. It may be a thematic issue but that is the nature of shards.
I personally think these 4 factors balance.