I saw all three of these, and I liked them all actually. I think some of the variations in reception are probably due to different levels of franchise fatigue, plus different expectations for each of these franchises in general.
There hasn't been a new Mad Max movie since 1985, so people weren't sick of the franchise. For many, it probably felt like a new series. The fact that it was made sort of as a reboot anyways (bye Mel Gibson, hello Tom Hardy) helped that perception. But the franchise as a whole has never really been rooted in continuity. The shift between the first and second movies is pretty big in my opinion. So I think that even though this one is probably in continuity with the first three, it feels like a new start anyways as the first new Mad Max in 30 years. It doesn't have to be in continuity if it doesn't want to, it can just exist on its own. The stories of this particular series have never needed continuity if they don’t want it.
Similarly, Jurassic World does exist in continuity with the other three also, but the second and third movies don’t really matter for its story. That says something more about those entries being disposable instead of anything about this one. It’s been a while since the last movie, so people were ready for another one, and the premise of World was a realization of an unfulfilled promise from the first movie: an operational dinosaur theme park. People who saw the first Jurassic Park twenty something years ago were getting the sequel they always wanted, and it was awesome.
The Terminator franchise... is completely different. It started with two great movies (T1, T2) and devolved into two meh movies (T3, Salvation) that are all within the same continuity. There was also a good, but complicated and prematurely cancelled TV show (Sarah Conner Chronicles). So before Genisys, there was a whole lot of Terminator to digest; people weren't exactly starving for a new entry in the series, and there was definitely an element of franchise fatigue in play. Given the complicated stories and the success of some of the earlier entries, a soft reboot (like Days of Future Past) was probably the best play going forward. And this story did work to reset the continuity, update the franchise, and honestly end the overall plot of humans vs Skynet if need be. Yay they finally stopped Judgment Day permanently forever! (If you ignore the post credits stinger.) I don’t think the film was unsuccessful because of how it approached continuity; I think it was because there just wasn’t a demand for a Terminator reboot yet, and because some of the story decisions in the third act didn’t make sense. It was still an enjoyable movie, but it wasn’t the right time for it.