It will stop a lot of attacks but if someone figures it out, you're screwed. So I don't recommend it.

But years ago I used the same password everywhere except with a few differences due to different requirements (like special characters) and the weakest passwords I used got leaked on pastebin (or similar). And sure enough many accounts got compromised, not a huge deal and I didn't lose anything I cared about.

The interesting part is that no-one seemed to try the leaked password + 1234 or a capital letter in the beginning.