I never thought about the risks of using finish from an old, previously opened can until I had to entirely strip a project because the ancient varnish I used didn’t dry properly.
Now I play it safe. I date every can when I first open it, so I know at a glance how long the remaining finish has aged. If the date shows the can has been sitting for a year, I test the finish it contains on a sample. Before I use any old finish on a project, I want to make sure it dries hard.
After two years, my freshness date expires. Using vintage finish may be appealing because it doesn’t cost anything, but buying a fresh can is a much better idea.