Informix does not have the concept of cold backups nor does it need it. That is only for those databases that can't keep there data consistant or figure out how to do real backups.
But anyway, use onbar when you have a storage manager, something like TSM. Otherwise you will find yourself setting up disk pools and managing your backups yourself. Onbar is the fastest way to backup and you canhave as many threads as your hardware can support. We saw back-up times reduced by 75%.
Use ontape to go to local disk or local tapes. It is single threaded. Also good for use with HDR.
I only use dbexport when I need to move a small database and it will lock your database.
As for onunload, well it was more useful before onbar and before row level recovery. I'm sure it still has it's uses, I just don't use it.