When I used to issue conditional line up clearances,there was always a problem with them.
You tell the pilot ''After the landing xxx line up xxx'' and they would readback ''Line up xxx''.
That happened many times.If you missed the readback (another Hot topic) then it was Tenerife/Milan time.
When our trail started I stopped using them,and found no real difference.As long as you had gaps provided from radar.Oh course you had the experts you would pack,and then try to launch outbounds.
Enough said.