i have recently put two Radians in our SUV and needed the angle adjuster or they would simply not fit. after putting them in i have no issues with them for a few reasons.
they really just act like a new top to the seat but only under the front and not the rear area, thus allowing the rear area to sit lower than the rest and thus changing the angle, since the pivot point for the seat is the rear area, the seat is just as tightened down with the angle adjuster in place as it is without it. you can actually put the seat in, tighten it down and then lift the front and slide the adjuster in almost. and then you tighten the backside of the seat that is forward with the top tether. it is rock solid.
after tightening it down fully both ways (without it requires me to move my front seats to a unusable position, but i did it once to see what it was like) it is actually more stable with the adjuster in place , since it is a smooth flat firm non slip base for the seat to rest on. it sinks in just enough to grab good and does not wobble. in contrast my actual seat had curves and the location of the latch system does not center the radian on the seat totally so it was a bit tippy.
i think i would use it even if i didn't need it, really