As far as I'm concerned, the two biggest problems server-side are "unable to load city at this time, please try again later" and not being able to take over a city due to the message that "you don't have all the content used in creating this city". Both of those issues were present from the beginning, and were never really resolved when it was actively supported. The "unable to load" appears to have something to do with a corrupted file on save, and if the corruption is on the server side, you're looking at needing a rollback. The "don't have the right content" error is, to me, poor programming from the beginning. Probably a combination of the promos and regional content that was not available to everyone, and then additional dlc's that are perhaps not checked properly in the scripts.
I'm not defending the existence of bugs, although I've noticed that many online games will have ongoing, unfixed bugs that the developers never address for some reason. I've learned to work around the couple "unfixable" bugs for my gameplay style. This may not be how you want to play the game, and that's understandable. I am actually glad that they still offer the game and the dlc's, even though there are a few things I wish they'd make available again. I wrote a post a while back that lists the different promos and country-specific downloads that you might need in order to take over an abandoned city; I don't know if it will help you, but fwiw, here's the post: https://answers.ea.com/t5/SimCity-2013/Request-to-EA-Make-Promo-AddOns-Available/m-p/6652082