I think code comments and breaking everything into well-named functions, along with ample side documentation, make it fairly clear to follow. The business logic gets confusing - but anyone who had to work on the code would need to understand that first anyway. I don't think the code itself is difficult. It's all in the docs. Frankly, I don't always remember how something should work and I need to spend a few hours to re-understand some tricky piece of 10 year old code again before I change it. That's the job.

My main concern, of course, is not leaving a mess for myself. That should be every coder's priority. Then taking over someone else's projects wouldn't be so hard.