From everything I've seen about lightsabers, they slice pretty handily through most materials (especially human limbs), but for some reason, in The Rise of Skywalker trailer, he bears his opponent to the ground like he's using a traditional melee weapon. Is there some property of lightsabers that allows them to push/repulse instead of cutting?
If you pause the trailer at 1:16, you can see that the impact point appears to be fairly low. In fact, it's plausible that Kylo has impaled the person (possibly a Resistance soldier) with the crossguard of his lightsaber and is simply pushing down against him using the lightsaber for support. Lightsaber hilts don't go through people's bodies very well.
Notice that a lightsaber's edge doesn't always have to cut everything it touches. Power is adjustable.
For example, training lightsabers generally won't decapitate nor dismember people:
Training lightsabers aka Youngling lightsabers were lightsabers of limited power—and thus limited danger—that were used for lightsaber practice. Training lightsabers were most often used by Jedi younglings before they underwent the Gathering and built a weapon of their own. It was also possible for normal lightsabers to be tuned down so they could be used in this fashion.
The blade power adjustment knob was a small knob located on the hilt of a lightsaber that could be used to make the weapon's energy blade more or less powerful, to meet the need of the current circumstances. Qui-Gon Jinn's and Anakin Skywalker's lightsaber had one of these.
Who knows, maybe Kylo Ren's saber was running low on power, or maybe he reduced the power (willingly or by accident). More likely though Robert is right and the guy in the receiving end of the blow wasn't touched by the larger blade of Kylo's saber.