Both of them try to mimic how CRTs work (though i think black frame insertion is closer) but as a side effect they affect the image brightness (usually causing everything to become dimmer) and can introduce perceptible flickering. And yes, some monitors are indeed marketed as the theoretical refresh rate you'd achieve with these effects in action - you'd need to find dedicated monitor review sites (e.g. i've heard some good comments about rtings.com, though ignore the scores and look for the metrics... their scores take into consideration stuff you often wont care about) and of course look for comments on places like Reddit's /r/monitors subreddit for people who already bought a monitor.

[0] https://twitter.com/System32Comics/status/131319474720949862...