There are a lot of opinions on this James.
I have called my insurance company before and asked if un-permitted work on my owner occupied house would void my insurance if it were to somehow cause a problem and the woman I talked to said no. I think that would be a big concern. Then you have the concerns Jennyo spelled out. How upsetting and expensive would it be to be told to undo the un-permitted work and put the house back together if needed?
How much would you enjoy the un-permitted stuff compared to the risk it poses. An example, if it were a little “California room” would it be easy to get rid of all the while knowing you probably wouldn’t have too.
If it the house having un-permitted work doesn’t bother you make sure it is safe above all, especially the electrical. Code is different. Thousands of house are not up to code as code changes every year. The un-permitted work on an older house might even be more up to code than other things in the house.
Lots of people buying fixers don’t care because they are going to do an overhaul anyway.
Last thought for now, is never pay for the un-permitted work and if you are interested in a property with un-permitted work you might even try to drive the price down with that information if possible.