WPUmbrella allows to send frequent reports to every customers, reporting the backups and updates made during the month, but also custom works. I can manage every updates on many WordPress websites through this tool. Also, the customer support is very good and reactive. I use it every week, and almost every day. It is very easy to add (through a plugin) and you can choose to hide WP Umbrella's plugin in the plugin list so it's unvisible for your client.

Cheap, does most things you need, good reports.

The maintenance reports are accurate and very comprehensive. I save valuable time each month sending them to my clients.