To me it didn't feel like there were many challenges or ways you could irreparable damage your city in City Skylines. You could maybe invent your own goals, like creating a specific look to your city or building the most density possible while maintaing efficient transportation. But that's what I'd consider sandbox: using your own creativity on a blank canvas with almost no chance of entering a complete failure mode you can't recover from.
The SimCity games felt more like they were a challenge needing to balance the budget and build a sustainable city all while being resilient to events like disasters. Might just be me, but I never felt like I could fail at C:S; whereas in SimCity you could make a few stupid decisions or expand too fast and start losing money, and all you could do is watch the city degenerate into a wasteland that can't be gotten out of (without cheating anyways). That made it feel more like a simulation.