You asked two distinct questions and it is important to separate them. There is a very big difference between
- editing your documents while updating;
- compiling your documents while updating.
It is definitely perfectly safe to keep working on your documents during an update. The only potential problems involve compiling them during the update. I avoid this since it could easily get messy. (If compilation failed badly, I might have to stop and clear out corrupted auxiliary files which would require me to compile again, rerun helper programmes such as
makeglossaries and then compile again. Nothing terrible but potentially annoying.)
The only problem with editing during an update would be if you had your editor configured to continuously recompile. In that case, merely editing would also be compiling.
It is true, as Keks Dose's answer points out, that even editing your document potentially confuses things in case of an error since it may take some time to determine whether it was your edits or the update which caused the problem.
However, it is extremely straightforward to take precautions against this. Keeping your documents under version control has many advantages, including the ability to return your source to a known-good state easily and without fuss. If you don't wish to use version control, though, just save a copy of the document before updating so that you can test compilation of that version if you run into problems.