It causes temporary and often permanent memory loss and brain fog. The goal of ECT is to, quite literally, make people detached from their illness or trauma by either forgetting it entirely or by damaging the pathways of learned stress responses, both of which are achieved through random damaging of soft tissue by routine 'treatments' until the right spots are hit, with every other damaged area being deemed acceptable losses.
It's proven effective, sure. It's more effective at causing improved mood than doing nothing. So is a heroin addiction. At least once you stop abusing heroin, you recover physically. ECT does permanent damage in most people who undergo it. Plus, ECT isn't a single course of treatments - any benefit it gives eventually wears off, and additional treatment cycles are planned in perpetuity. This is because of neuroplasticity, where the brain will recreate some of those connections that were damaged by ECT, thus bringing back trauma memories/associations and symptoms of illness.
Is it really worth it to suffer bits of permanent damage every time you undergo what is essentially medicalized repression of memory? There are people who lose memory of their partner of several years, have zero emotion towards them, leave them and continue on in their life never remembering the love they had. Is that worth it, for something that just returns anyway?
"Proven effective" and "outperforms placebo" are statements that focus on a single variable and don't mention how much damage something may cause elsewhere. Don't take it at face-value.