Some light spoilers then
Metamorphosis is a very grounded story about a girl whose life took a nose dive after being introduced to drugs, and as implied by DundaStation has a downer ending in the original version.
The common joke is that gooners looking for a doujin to fap to end up being emotionally broken -- which personally I didn't get but to each their own -- but because it's so grounded in reality it can set some people off, especially if they've had past trauma.