I have two Merlins.

Merlin unit A: dovetail plate attaches tightly to the Merlin platform, and tightly to the three tripod adapter plates I own.

Merlin unit B: dovetail plate attaches tightly to the Merlin platform, but is loose on all three tripod adapter plates. I had a DSLR mounted to the tripod with the adapter plate, and didn't notice it could slide. I shot a couple of hours like this. Unfortunately it is not obvious it is loose at first glance, it seems okay when the head is level, and I wasn't looking for a problem. If I had tilted the tripod head further forward before noticing, the camera would have fallen off.

I see no adjustment possible to the lever on the dovetail plate. The lever on the good unit A requires more force to move than unit B.

Comments/suggestions, please!

I just place a small piece of cardboard on the adapter plate for a tighter fit.

with time Merlin's lever get's loose, when I had it I used gaffer tape

Yes, tape is how it will be dealt with.

My post was more of an alert to others to avoid assuming how things will work with this unit, which is perhaps more prone to be viewed as a higher quality product "without issues". You are relying on friction to hold your camera in place with this design, the friction must decrease with use, and there is no safety catch in the design as a backup.

Have you contacted Tiffen?

Not yet, have been busy. The problem unit is an older one, on my newer unit, the plastic turn knob that does the clamping appears to be from a different mold. Maybe this is an old issue they have taken care of.