IchoTolot used cheap trick........quite easy to fix for Nepster!

...or maybe not .

Honestly, I am starting to think about returning back to V3 and calling it the final version.

Better, but you still used only one of the two tricks. V3 will come tomorrow.

Playing a recently released level, I (re)discovered another guideline: Be careful with multitasking.
If your level needs multitasking at several different and far away places, this might easily lead to player frustration. This holds especially in the following situations:
- Requiring multiple skill assignment within a few frames: Players are then forced to frequently jump from one place to the other and back again, because they won't know which lemming needs the skill assignment first.
- Levels with vertical scrolling: In such levels, the minimap at the bottom does not nearly cover all of the level. Therefore players cannot use it to check how far lemmings in different locations are.
- Having multiple hatches and requiring early skill assignments at several of them: The problem here is, that players do not know the order of the hatches. If the level is covered by the minimap at the bottom, this migitates the problem. But it is definitely annoying when combined with vertical scrolling.

Here is an obscure low-priority bug: Pickup-skills, zombies and ghosts appear only at the second frame. So when pausing at the first frame, the player will see a rather inaccurate version of the level.

I'd like to see some recent disjoint unions, along with the reasons why they're deemed good levels.

Not really recent, but here some good disjoint unions:
1) Oh, the possibilities... in shveg03.dat
The level is split into four disjoint parts, but the skillset is very restricted. So you have to decide between saving all lemmings from one hatch (which one?) or saving some lemmings from multiple hatches (again which ones?). And only very few of your choices will actually work...
(The actual solution is not that interesting, but that is due to the design of the pieces, not because it is a disjoint union.)

2) Everyone But The Bomber in twbestof.dat
Again we have four disjoint parts. Each part in itself can be solved in many ways. But the challenge comes in distributing the skills and choosing the correct solution for each part.

3) Multiple Choice in twbestof.dat
Similar to the level in shveg03.dat: Four disjoint parts and one has to choose now three out of four to save. The time limit is annoying, but otherwise...

4) The Final Countdown in GARJEN06.dat
Mirror design, but different solutions for both sides required (due to skill restrictions).

On the other hand, there are disjoint unions that this fails to describe, because they seem to have been designed as disjoint unions from the get-go -- Fast Food Kitchen and Rendezvous.

I am not so sure with Rendezvous. It looks very similar to some early levels of mine (which fall squarely in the category of bad disjoint unions), and they were typically created in the following way: After one side was done and playtested I realized, that there is some more space available. So I filled it with the second half of the level .

Let me add just my 2ct to this discussion.

The "Good examples" in the first post are bad examples.
They are indeed very good levels, but they do not fit the topic: They are not disjoint unions! In (at least my) solutions both sides interact, so they share the mirror design seen as well in Fast Food Kitchen, but this is as far as the similarities go.

Synchronized Lemmings is a good level for ONML, but not for NeoLemmix.
In L1 and ONML the difficulty in executing multitasking is a deliberate part of the game. We may consider this good or bad, but that is a topic that is independent from disjoint design. Once you accept that L1/ONML was meant to have levels, where the main goal was to succeed in executing an easily found solution, then levels like Synchronized Lemmings become indeed very good levels, in the sense that they feature this part of the game in an interesting way.
On the other hand, NeoLemmix with all its features removes this part of the Lemmings game (again as a deliberate design choice). Thus it removes the sole reason, why levels like Synchronized Lemmings were included in the first place. So it is no wonder that the level that remains is seen as a bad level.

On the other hand, I totally agree that levels like Rendezvous at the Mountain are bad levels even in L1. And I agree as well, that You take the high road would have been a much better level for ONML, if one half would have been mirrored.

Disjoint unions are not the real problem here.
Coming back to the topic at hand, I feel that disjoint unions are not the real problem. The real problem is, when level designers feel that an uninteresting level can be made better (or more difficult), by prolonging the level or adding further hatches. This does not help at all! On the contrary: If makes the level worse, because the new level will not be more difficult, but simply more annoying.
Of course, frequently such prolonged levels will coincide with disjoint union levels, but that does not mean they are the same. There are some disjoint unions that are good puzzles (e.g. by having to figure out the skill distribution), while there are some bad prolonged levels without a disjoint union design.

This is seen At least the variant for builders is seen somewhere in the game: Builder loses ground by terrain-remover. The builder will continue to remain in mid-air, and move onto other air pixels.

Good catch. And I realized only now, that a stacker will continue stacking, even when the terrain, he is standing on, is removed. Until namida's remark, I automatically assumed that stackers would fall down and their work is interrupted. Actually, what is the reason behind letting stackers continue their work?