Unleash is bad because it has more inherent drawbacks than similar alternatives.

tl;dr

Replace "seal gain frequency" with "total time to gain maximum seals" (aka
'sealing time'). This makes unleash more unique, and more usable.






Details

(1) Adding more maximum seals to unleash makes casts slower
(2) Adding more maximum seals to unleash makes gaining maximum seals slower
(3) seal gain frequency is extremely exotic affix-wise

Unleash itself is generally worse than spell echo, mines, and traps.

Adding more maximum seals to unleash already makes casts slower, and provides almost no benefit because PoE is DPS based, not average hit based.

The correct fix is to replace seal gain frequency with "sealing time", where adding maximum seals does not change how long it takes to reach maximum seals.

e.g. gaining 5 seals takes 2.5 seconds.
You add 5 seals.
Gaining 10 seals still takes 2.5 seconds.

This makes max seals an attractive way to scale unleash.