I just had my very first online conference a couple of days ago. I held a poster presentation (you can see it here).
The experience turned out better than I thought. It was very well organised and I learnt a lot; so, yes, it was very useful. All sessions ran smoothly because they all had a moderator and a member of the IT team to address any problems. Although I would agree that online conferences might not replace the feeling of going to a new place and meet other researchers personally, it can also be benefitial for researchers who can feel overwhelmed by the event but are better at building connections through messages online.
In a nutshell, like in many other spacts of life, organisation is the key. It includes not only organisation before the event but also during the event and being ready to address unexpected technical issues promptly.