Oh, dear... Yes, I have been down this road before (old employer). If you are going to have multiple clients on the same box, do absolutely make sure they are on separate instances. Especially if you're dealing in insurance or health services software. HIPAA will bite you big if you get client data mixed up.
Do not, under any circumstances, put them on the same DBs / same instance regardless of what services you are providing. It will be a maintenance nightmare, not to mention a legal one if one client gets ahold of another client's data.
So, Pros... Easier to watch one box than 2 or 3. Physical space in the server room is saved. Utility bills are lower.
Cons... Unless the instances are on Virtual machines (which may or may not work for you), security assignments can get mixed up (again with one client accessing another client's data), a hard drive crash (or any hardware problems) affect all clients on that box, SQL Agent Jobs will multiply because you'll need one for each client / instance on the box, and data theft (or server theft really) is potentially a bigger headache.
Not sure if maintenance is easier or harder in this case. It's sort of half a dozen and 6, if you know what I mean.
Biggest issue, in case I haven't said it enough, is the potential for cross-pollenation of client access / data. You need to take this implementation slowly and be absolutely sure everyone knows which logins and roles and jobs are for which instances and that you absolutely idiot-proof the servers from internal security issues. I've seen more problems caused by a well-meaning dba / developer who ran something on the wrong instance or gave someone the wrong information because they were looking at the wrong client db, etc.
It is crucial that you get things locked down so that only one or two people have access to make changes once things are consolidated. And that the setup is documented to death and everyone knows where the documentation. And that there is plenty of redundancy and high availability for your setup.
Does that help? Or did I just terrify you?