Where do you write yours down?
It works pretty well. Different password for each site, I only have to remember a few things, and it would take several compromises (and a weirdly dedicated attacker) to work out my algorithm.
For example, my 'insecure' fixed part might be 'Tenk5$' (I recommend including uppercase, lowercase, a number and a symbol in that part to get around idiotic password requirements). Then my algorithm could be 'the last 5 letters backwards, skip the first vowel'. In which case my password for HN would be 'Tenk5$rtani'.
Some need Lower-/Uppercase, some with numbers, some with special Chars, some restrict to minimum of x chars, some use a maximum.
Your system works not for all things, i use a similar system, but store a bunch of passwords with last pass. Only really important passwords are in my head.
I keep meaning to collate idiotic password rules so you can see for sure which patterns work in all of them, but I've never got around to it.
That has not worked well for me. I have lost a small amount of bitcoin, and some encrypted homedirs and encrypted hard drives.
Use that algorithm widely enough and several compromises are guaranteed. The only thing protecting your password is that url manipulation.
It stores your data locally, is suprisingly easy to use and relies on battle tested GPG.
From the website: "The community has even produced a cross-platform GUI client, an Android app, an iOS app, a Firefox plugin, a Windows client, a pretty Python QML app, a nice Go GUI app, an interactive console UI, Alfred integration (1) (2) (3), a dmenu script, OS X integration, git credential integration, and even an emacs package."
The lack of integration then with the rest of the OS is actually a security benefit.