How do the Elac Uni-Fi UB5s compare to more expensive bookshelf speakers?

Even tougher. Just found a used pair of the finished slim cabinet Elacs for $600 on Amazon. I could get those tomorrow.

At or around your price you can get a previously owned pair of Revel M22's . Reference quality sound for a very affordable price.

If you have the funds to get the ELACs from Amazon now, then I'd do that and take a good listen for yourself. Amazon has an excellent return policy so you've got 30 days to get a feel for whether they are for you (in your space, room matching should not be overlooked!). If not, return them for a refund and save up for some KEFs. 

I own the Wharfedale 10.1 and 10.2.i heard the Ls50 with ML 100 w, accompanied by audio research preamp and theta transport n dac with aq diamond ic n dragon sp cables, they blew me away, this speakers are capable of giving you anything you connect to them, there's limit of course...
The Elac UniB5 I heard at Axpona last year, connected with Audioalchemy gears n Audioquest cables, they also sound very very good.
So which one to choose? If you have budget and good gear Ls50 is the way to go.this is a quality speaker...well made, well built...The B5 you can't go wrong as well...no matter what they are musical...

Another point to keep in mind is the cost of the stands. They tend to make a noticeable difference in the sonics so you don't want to skimp when buying a $1500 speakers.