If you knew for an absolute fact that *every* person on your guest list is tech savvy and has daily access to a computer and internet, would you consider doing online RSVPs? I'm not concerned at all about the website not working - I'm marrying a software engineer and I have absolute confidence that whatever he put together would work without fail.
FI's family and friends are all software engineers or IT people, and the few who aren't are tech competent anyway. I frequently get emails and links to reddit articles from his grandmother. My own grandmother created and runs an online business.
One benefit of going this way is that we can make it impossible for people to add additional guests to the RSVP, but I'm still not quite sure.
Thoughts?