I wouldn't use premises at all, but the more generic site:

Virtual events are less effective than on-site.

But if you had to use premises, perhaps the least bad option would be:

Virtual events are less effective than on the premises.

I wouldn't use premises at all, but the more customary onsite:

Virtual events are less effective than onsite.

But if you had to use premises, perhaps the least bad option would be:

Virtual events are less effective than on the premises.

I wouldn't use premises at all, but the more generic site:

Virtual events are less effective than on site.

But if you had to use premises, perhaps the least bad option would be:

Virtual events are less effective than on the premises.

I wouldn't use premises at all, but the more generic site:

Virtual events are less effective than on-site.

But if you had to use premises, perhaps the least bad option would be:

Virtual events are less effective than on the premises.