I'd say it's a lot of fun.
Tangentially related for some of the younger devs: The predecessors on your project probably weren't idiots. It's possible, but they were likely working with a different set of requirements under a different scope. Even the code that it morphed into over time was probably a good enough decision for the constraints at the time.
At work myself i work mostly in isolation on codebases that have been in maintenance mode for years, where the original authors have typically either left the company or moved to another dept, and where there is almost no documentation. Everything is hacks and exploratory work, but it sure ain't fun. Adding relatively uninteresting small changes can take days or weeks.
Working on a broad and deep project as half of a pair is my preferred working mode so far in my career.
I'm lucky to be writing fun code at work - most of the time -, but the side projects keep me sane for many reasons. Not having to deal with 'bad' code that I didn't write, is also a nice benefit. All the bad code in my personal projects is mine, so no one else to blame :D
EDIT: typos and delete possibly negative statement
EDIT: I have deleted the original statement. After your comment it made me think that it might involuntarily scare people from sharing things in the open source world, which is not my intention.
But the author said "in my career," which I took to mean during the course of ones paid employment.