Multisite scablabity questions

Hi there mates down under! We are thinking deeply about Multisite for use i a project that could have a lotttttt of clients.
What are your experiences as far as pushing big numbers like 500s to many thousands of different sites and customers. Good news the sites are going to be very simple landing page sites . Other fact each client could have more then one landing page would prefer a common back end.

1 Is that even reasonable ?
2, what addition tools like support dash board tools etc.
3. Maybe we should look at a custom development