Build or Buy? When should you create your own background job system?

Hey everyone, so I've been chewing on the idea of background job processing. We're currently using a managed service, and it works okay, but I'm starting to question if building our own system would be better. Are there specific scaling issues, or customization needs that would make building our own worker/queue system a good idea, even if we have to maintain it? Or is it usually best to just go with what's available unless you have truly weird requirements?