I don't think that any of the ZIF carriers actually have a CPU frequency limit. There's a limit to have fast of a clock signal they can supply to the CPU, but what the CPU does with it afterwards (multipliers) won't be (entirely) controlled by the carrier card.
That said, that limitation was probably written by marketing, back when the top CPU:bus multiplier was 10:1. So XLR8 figured, "well, we can supply a 50MHz clock, and the highest clock mulitplier is 10:1, so our top speed must be 500 MHz." And the jumper/switch settings for the clock muiltiplier are on the carrier card.
The above does not reckon with the fact that later CPUs redefined some of the clock multiplier codes/pin settings to mean different, higher multiples, nor did it take into account the chips which are capable of a soft multiplier setting (PPC750fx, PPC750GX).
In my opinion XLR8 probably had the best, most developed carrier card technology, but ultimately, a carrier car is pretty simple. It must supply a clock signal (generally between 40 and 50 MHz (although up to 65MHz can be useful), it must have onboard power regulation that honors voltage level pin settings from the ZIF, and it needs to have switches/jumpers to properly bias the CPU clock multiplier pins on the ZIF card.
PowerLogix made a smaller, simpler carrier card that seems solid. I think Formac also sold one. And NewerTech sold a few as well.