Most of them are map based and/or have the same heuristics Tesla previously had (basically a limit for how much and how rapidly the speed can be reduced).
I'll give a concrete example, for example Ford's speed limiter system can't apply the brakes, it can only do engine braking. That drastically reduces the available deceleration.
Depending on the point of view, Ford's new speed-limiting technology either helps lead foots curb their appetite for speed or represents a further intrusion on driver control and automotive privacy.
www.americanautomakers.org
However, when you use Blue Cruise, where it is allowed to use the brakes and it works based on sign detection, exactly the same thing happens:
"The truck will see a change in speed limit sign and
harshly apply brake where maybe the speed limit dropped from 70 to 55 for a construction zone"
NHTSA was specifically not happy with how Tesla's system responded to lower speed limits as part of the FSD investigation. Note the NHTSA does not universally apply findings to all manufacturers or to all systems. For example, Infiniti's earliest lane keeping system didn't have attention detection, but because it never got a NHTSA investigation it never had to add them.
And in a similar vein the same thing may not apply to vanilla AP. In some areas, there are some mapping errors, and AP would drop the set speed, but the car never applies the brakes, just applies some regen, which is easy to override with accelerator. Same for when set speed is much higher, it doesn't floor the accelerator, but applies a limited amount (such the sometimes I also override with accelerator).
As another pointed out right above, in some roads (especially construction zones that may not be mapped) to meet the reduced speed limit, you really actually have to apply the brakes. This is because by the time you see the sign, if you need to be below the speed posted, letting off the throttle is not enough. The only way to do a gradual slowdown is if you were aware of the limit before you see the sign (or there was a previous sign that warned of an approaching zone, which I'm not sure any system out there recognizes at the moment).