Hi Deborah,
You asked some great questions I hadn't thought about - argh! I have Netflix and Hulu set up on each Roku. I haven't had any problems. I have my own personal Netflix, Hulu, and Sling on the Roku. There is no pay-per-view charges, so it is the flat monthly fee. We have guests that logout of my Netflix and use their own (probably so they can continue watching what they had viewed?). I called Netflix and there's no easy way for them to hack into the system and they seem to think it was relatively safe. We added Sling to our account after the innauguration when I realized there were some things you really need to see live and not the next day:)
Netflix limits the number of accounts you can sign into and use. We have had guests that have left their account on the TV and we just logged back into our account and it wasn't a problem.
Personally, we love Roku and our guests seem to like it too.
Chavonne