That's a huge benefit though - I'm more than happy to slowly and methodically refactor a system if at the end of it I can be nearly 100% certain that I haven't

That's a huge benefit though - I'm more than happy to slowly and methodically refactor a system if at the end of it I can be nearly 100% certain that I haven't inadvertently broken something in the process. The time spent upfront is multiplied time saved in fixing bugs in production. In a tortoise and hare race, sprinting off quickly doesn't count if you can't reliably reach the finish line.