Leveler blocks used as recovery boards

In an emergency I would use them. If I was planning I would carry something else. I bought a folding mat that you can toss down, more for me on ice then mud or sand.

My yellow lego ones have broken on gravel, so I don't think they are strong enough.

The open web "lego" ones are plastic and can get very slippery in mud and water. Sometimes just turning them upside down so the nubs are not on top, may make for more surface area for the tire to grip. But if it's just too muddy, too lightly soft sand, or to much water, they may be best used to sit on until the recovery vehicle arrives.

They might work in certain conditions and if you are not stuck too badly. We tried once to use the blocks to get unstuck in very wet, deep mud. Even with the knobs on the blocks, the tires couldn't get enough grip to get out of the hole. All that slick plastic was, well, slick. Had to use a ton of collected rocks to line the way out instead.

Not so much, as they require the firm support under them and as an assembly aren't resistant to bending moment.
I'm sure they would help in a bad stuck, but you'd probably be digging them out of the bottom of the tire ruts afterwards.

I have seen recovery boards shoot out from under a tire, was glad I was not behind it, needs some grip on the bottom and the top.

In an emergency, anything on the rig can be used. If you use the lego blocks on an uneven surface there is a good chance they will not operate as designed again. In other words, they might break. The other side of that is there is a better chance your tires will get a grip on the lugs sticking up on the lego block than there is on a smooth muddy board.
I've used them when entering a "pull through site" with my 5th wheel that was terraced with a retaining wall on the lower side and not enough turning radius to get the camper wheels around the corner behind the wall. I built a ramp with them just like If I was using them to level one side so I could pull the trailer wheels up over the end of the wall. No reason you couldn't do the same thing if you need to get the wheel of the truck over an object or out of a hole.