personally the biggest problem i have with pepsis is just its potency. pepsis is functionally a 150 potency aoe heal, because the difference between regular prognosis and e.prog+pepsis is only 150 (300 vs 100+350).
getting a non-tank to full health needs around 2300-2500 potency. 150 is a drop in a bucket.
you must also consider the times when pepsis is used. the situations where a sage resorts to pure gcd healing are not only rare, but also dire. with so many other healing and mitigation actions, the only time youll need to use pepsis is when you have already exhausted everything else. 150 total extra potency is far from adequate for a last resort tool, and even less one that can only be used once because of its cooldown.