The problem with -Werror in open source projects is that it can produce errors for users building your code on different compilers or compiler versions than the one you originally developed it for.
As compilers change they can end up adding new warnings to the defaults, or for or example -Wall, or -Weverything. And different compilers might throw different warnings on the same code with the same options involved. Using -Werror will force an error, which can end up terminating the build for users, even when the code compiles cleanly on your version of your compiler.
So -Werror is fine if you are shipping binaries, but if you are shipping source code, it's probably a good idea to not use it in the public build system.
What's the difference if compilation stops at the warning? You'll (or your team, or whoever) fix it anyway, and if you won't, you have a people problem, that must be solved at the policy (or HR) level.
Solving people problems at the tooling level is a certain way to get unintended consequences and alienate the good people that weren't part of the problem. Of course, you can get some tooling to support your people, but tools to police them are worth less than zero.
Anyway, unrelated to that, I do like to live warnings on code that is not ready to production. It's an easy (effortless in fact) way to make sure it'll be fixed before deploying.
-Werror is a cheap unavoidable automatic code review tool that limits the blast radius of what you call "people problems". It slows good programmers down a little but it can stop the dangerously bad ones in their tracks.
As compilers change they can end up adding new warnings to the defaults, or for or example -Wall, or -Weverything. And different compilers might throw different warnings on the same code with the same options involved. Using -Werror will force an error, which can end up terminating the build for users, even when the code compiles cleanly on your version of your compiler.
So -Werror is fine if you are shipping binaries, but if you are shipping source code, it's probably a good idea to not use it in the public build system.