Maybe it's different in other languages, but if I ever see hand-rolled serialization in a C# project I'm putting replacing it at the top of my to-do list. There is pretty much no reason to do that ever, serialization is a solved problem. It's flat out stupid to spend extra time hand-rolling a solution that most likely ends up buggy and requires even more work in the future etc. I also often see people waste hours or days writing custom serialization logic for a serialization library, when they could have just written a new class in the shape of the data and been done in 5 minutes.

I did work on such a project once. There was a CMS web app which needed to sync data from a third party api to rebuild some pages every night. To do this they had a client library that pulled the data twice daily and stored it in a db, then the CMS app pulled the data through the client library. The API used XML and significant parts of this library were a hand-rolled xml serializer. It worked so I didn't touch it, then it stopped working. Spent days trying to fix it but the more I studied it the more I realized it was entirely pointless. Spent about 1 day replacing the whole thing with a simple little function that just calls the api, deserializes the data using a library and builds the pages. Remove the pointless DB, remove the giant legacy quagmire integration library, this project probably took months to write and I replaced it in a day, making it orders of magnitude smaller and faster.

In the end my solution wasn't used because of office politics, apparently they'd paid a significant amount of money to use this pile of trash and replacing it that easily would make someone look bad or something. So I ended up fixing the pile of crap in the end, even though I had a replacement ready to go.