Smart motorways are where most of the variable limits are, that's why I mentioned them.
The problem is that too often they cry wolf. You get a 20 limit, or even a full stop, on a motorway due to some dire risk ahead. But then it turns out there was nothing, or a car drove the wrong way down a slip road for a few seconds before realizing and turning around.
Many people ignore it, so if you do 20, or stop, you have people zooming past at dangerous speeds and risk being hit. The police can't really prosecute you for ignoring it, because your defence will be that it was unsafe due to everybody else also ignoring it.
Much more common is the random 50 limit for no reason, or they forgot to take the signs away after finishing work, or they left the signs up for months before coming back to finish something off. It undermines the whole system and people who do the route regularly and know there are no cameras ignore the limits. The whole thing needs a re-think.