My local library service in England has had the "Open Access" scheme in place since before the pandemic.
I don't think there have been any major problems with it.
You need to go through an induction course on things like fire safety, then you use your library card and PIN to enter the building out of staffed hours.
It's very convenient to be able to go at any time from approx 08:00 to 19:30 every day.
The "tailgating" problem is a little overblown - you're encouraged not to let others in, but they know that you can't do anything to stop it, and they don't expect you to put yourself in any danger.
There are still staffed hours throughout the week, and these work on a sort of rotation so that a neighbouring library will likely be open if your local one is unstaffed.