It's a very important but very subjective measure, for which I think that subjectivity belittles its importance.
Perhaps the more intriguing idea is that for each level of difficulty of use, there exists some amount of value to be gained wherein that weapon becomes a viable choice, and that moreover this value differs based on player skill level among other factors. There's also skill as it pertains to individual weapons to consider; if I'm skilled with acheron, tortofists have to offer something pretty good for me to forget about my training and jump on the tortofist ship.
It seems like a pretty good ranking.
However, there are multiple ways to measure easiness. Take Antigua-line guns for example. In some ways they are easy (many fast bullets in regular attacks) and in other ways hard (charge attack locks you in place for a long time, low damage means longer fights and more opportunities to get hurt, etc.). And alchemers are not very hard, if you're not trying to get huge DPS.
I'd say that your ranking is something like: "For whichever has higher potential --- regular attacks or charge attacks --- how hard is it to reach the weapon's potential?" Let me know if you disagree.