For those who started out on Z80 machines running Citadel on dual floppies and dealing with constant BDOS errors, moved to Citadel in x86 and x64 platforms, migrated to Citadel/UX - today's Citadel is pretty amazing, without a doubt.
Citadel from back then is pretty much responsible for my career - but I wouldn't hesitate to use it in a large environment for groupware collaboration today. In fact, I think it is far easier to get your head around than Sharepoint.