I think the sibling answer by oasisaimlessly is really good. I'd supplement it by saying that because you can have the entire configuration in a git repo, you can see what you've changed at what point in time

I'm the beginning I was doing one change, writing that change down in some log, then doing another change (this I'll mess up in about five minutes)

Now I'm creating a new commit, writing a description for it to help myself remember what I'm doing and then changing the Nix code. I can then review everything I've changed on the system by doing a simple diff. If something breaks I can look at my commit history and see every change I've ever made

It does still have some overhead in terms of keeping a clean commut history. I occasionally get distracted by other issues while working and I'll have to split the changes into two different commits, but I can do that after I've checked everything works, so it becomes a step at the end where I can focus fully on it instead of yet another thing I need to keep track of mentally

I just realised I didn't answer the first question about what keeps me from discovering the issues earlier

The quick answer is complexity and the amount of energy I have, since I'm mostly working on my homelab after a full work day

Some things also don't run that often or I don't check up on them for some time. Like hardware acceleration for my jellyfin instance stopped working at some point because I was messing around with OpenCL and I messed up something with the Mesa drivers. Didn't discover it until I noticed the fans going ham due to the added workload