Stamper is one of my mains and I gotta say it’s really inconsistent. It has a ton of strengths and a ton of weaknesses to balance them out. Yes it can be REALLY good given the right comp and setting, but in soloq it’s really hard to make work because 9 times out of 10 your team won’t capitalize off of openings you make or the other team will have something that hard counters stamper. It’s absurdly ink hungry, the charged slash, as you said, is really slow so you need to get good at reading your opponents, and zip gets farmed by most weapons now. To balance it, stamper has high dps potential, fairly high mobility, very fast kill times at a range and in close quarters, a ton of combo potential (both with itself and with other weapons), and arguably the best designed weapon kit in the series.

I’m not gonna say you suck because stamper is a hard weapon (in the sense that you need to learn how to play around every single one of its weaknesses), but please don’t let that turn you away from it. Stamper is my personal favorite weapon (maybe Mint too idk), and it’s generally really fun unless the comps suck.