ok, so I spent time on this last night, examined joints under magnification, etc. I do not believe the metal joiner inside each Unijoiner exerts any upward pressure, the 'floor' of each metal joiner has clearance under the rail.
I found that applying light pressure to the rail which is sitting high works pretty well. I took the tip of a flat bladed screwdriver, and pushed down on the top of the rail, and made most of the joints near perfect. I found you can overdo it though, it is actually bending the rail a bit so light pressure is all that is required.
I'm awaiting arrival on a new ON30 railbus and will test the track this weekend.
I found something else under magnification, you can see the rail ends are sawn by Kato, there looks to be tool marks, and on most rail ends there is a distinct burr at the end going vertically. I deburred a few sections using a stone and the joined joint is smoother. It does not take care of vertical mismatch, but it eliminates the burr.