ok here are the fixed levels  Taxing "81 Rainbow Road" and Mayhem "104 Where's your Holiday Spirit?"

81 Rainbow road
     The radiation objects had to be removed(replaced with fire objects) since in New formats there are no radiation objects(which makes the level not open).
     also had to replace the old teleporter objects (also makes the level NOT open) with the new format ones (the teleporters now in new formats say "IN" and "OUT")

104 Where's your Holiday Spirit?
      many objects had to be fixed to work with the new formats version. That's why the level didn't open in new formats. level is now fixed.
      Also add the rock-christmas style to your new formats folder (So level will open)

again here are the 2 fixed levels and the rock-christmas style (which should be added to the new formats folder).

rock-christmas style is a mix of gigalems' "gigalem_xmasrock" and  the "ohno_snow" style

Also updated the OP with the fixed levels and posted the rock-christmas style(See reply #19)

The FUN level "19_Bash_Left.nxlv" has an issue with the trap at the left. The trap is a tad too high and the lemmings walk right over it with no death.

I built a ramp to the trap and you can zap a lemming by dropping him right on the trap from above.

I've never edited a level. I'm hoping someone familiar with the editor can fix it. I can update the zip file again if I get a fixed level.

There could be more issues with later levels after a full play-through.

I should add that playtesting the level is absolutely not necessary to check if the trap works All you need to realize is that as long as part of the trigger area is slightly buried in the terrain, it will work! By extension and the same reasoning, even when exits sit directly on the ground, they might not work as intended because none of the trigger area is buried in the terrain. Therefore, the best way to ensure that exits/traps work is to simply make sure part of the trigger area is inside the terrain. As long as this happens, they will work.

The reason why the ghostbuster trap wasn't working before was how all of the trigger area was on or above the terrain. In other words, the trap was one pixel too high, so therefore it just needed to be lowered by one pixel in order to function. This is the very nice thing about New Formats NL To check, simply toggle on "trigger areas" in the editor, and all objects with trigger areas will be highlighted in pink.

In the 1.43nc editor, trigger areas are only shown if you select Lemmix/Neolemmix as the style when opening a .lvl file. If you select Superlemmini as the style, they will not display. In any case, trigger areas work differently depending on the platform/engine, and if the traps function on Lemmix/NL, they should work on Superlemmini as well and hence no adjustment to the traps is needed. For example, in Lemmini the trap checks for the middle of the Lemming, while in Superlemmini, the traps check at the foot of the Lemming instead, and as has been mentioned at times, while the latter may seem odd, this is actually how trigger areas work on Dos/Amiga. Hence this explains why it's much harder to avoid traps on Lemmini but in contrast much easier to avoid on Superlemmini by, say, building over the trigger area.

I just checked Fun 9 and the same problem with the ghostbuster trap occurs there as well, where it is placed a pixel too high and hence doesn't work. Therefore, the trap not working on New Formats NL is simply due to the conversion preserving that. Makes sense, since the pack was originally for very old Formats NL.