I think it's okay with SoF having lower costs than other PC, because SoF is a SHARD. If it has to be balanced with regular cards, then other shards like SoR, SoP, SoFr need to be nerfed as well... Furthermore the shard can be countered by CC before it can make its full effect, while Explosion and Steal don't have this problem.
To make it benefits more to

than

, Making the BH aspect

only seems to be a good solution:
"The shard is destroyed when HP>49, and if your mark is

BH effect is triggered"
or
"If your mark is

, using Accretion absorbs 1 quantum per element from the opponent. The shard is destroyed when HP>49"
What really is OP is the whole concept of PC. That's why the shard is considered OP.