-only being able to modify preplaced units
Nobody has made this claim, as far as I'm aware, and I'd have corrected them if they had. I mean, EUD Actions (EUDAs) can modify things other than units, so it's demonstrably untrue, but they can also modify properties on the unit type, which would change values for every unit of that type.
-(sometimes?) requiring an eud enabler
Bingo. I've been told Fish server has an enabler built into it, unlike the regular Battle.net servers. On a regular server, the SCV does a normal attack (unless you run an action enabler, in which case, the EUDA setting its attack to glave wurm runs). There's no mystification here: it works exactly how we've told you it works.
To clarify the "sometimes?", since you seem to be confused:
EUD Conditions (which we usually just refer to as EUDs) do not require an EUD Action Enabler. They will work as-is.
EUD Actions (which we sometimes shorten to EUDAs for differentiating from regular EUDs) require an EUD Action Enabler. They will not do anything otherwise.
EUPs (Extended Unit Placements) can be used to achieve the same effect as EUDAs, but they're tricky to handle. See
A Practical Guide to EUPs for more info.
so what exactly is the deal?
Here's a quick dump of the EUDAs the map is using:
Set Deaths("Player 3", "Int:18432", Add, 1280);
Set Deaths("Player 2", "Int:17416", Add, 768);
Set Deaths("Player 2", "Int:17475", Set To, 1);
Set Deaths("Player 4", "Int:17474", Set To, 16777216);
Set Deaths("Player 7", "Int:17474", Set To, 65536);
Set Deaths("Player 10", "Int:17474", Set To, 1);
Set Deaths("Player 4", "Int:18611", Subtract, 39);
Set Deaths("Player 10", "Int:18535", Subtract, 16);
Set Deaths("Player 5", "Int:18587", Add, 4);
Set Deaths("Player 9", "Int:18625", Add, 16777216);
Set Deaths("Player 3", "Int:18318", Add, 1310720);
Set Deaths("Player 2", "Int:18535", Add, 2560);
Set Deaths("Player 11", "Int:18535", Subtract, 8704);
Set Deaths("Player 5", "Int:18536", Subtract, 720896);
Set Deaths("Player 6", "Int:18536", Subtract, 3145728);
Set Deaths("Player 5", "Int:18536", Subtract, 40);
Set Deaths("Player 7", "Int:18536", Add, 637534208);
Set Deaths("Player 1", "Int:17472", Add, 9);
Set Deaths("Player 5", "Int:17474", Add, 1);
Set Deaths("Player 11", "Int:18587", Add, 32768);
Set Deaths("Player 6", "Int:18587", Add, 4);
Set Deaths("Player 9", "Int:18537", Subtract, 50);
Set Deaths("Player 3", "Int:18535", Add, 587202560);
Set Deaths("Player 4", "Int:18536", Add, 9999999);
Set Deaths("Player 5", "Int:18592", Set To, 402718720);
Set Deaths("Player 7", "Int:18536", Add, 458752);
Set Deaths("Player 9", "Int:18612", Subtract, 25);
Set Deaths("Player 5", "Int:18536", Subtract, 16128);
Set Deaths("Player 2", "Int:18611", Subtract, 70);
Set Deaths("Player 6", "Int:18611", Subtract, 1409286144);
Set Deaths("Player 11", "Int:18236", Add, 16777216);
Set Deaths("Player 7", "Int:18432", Add, 1280);
Set Deaths("Player 10", "Int:18625", Add, 16777216);
Set Deaths("Player 5", "Int:18318", Add, 1966080);
Set Deaths("Player 2", "Int:18611", Add, 100663296);
Set Deaths("Player 5", "Int:18611", Subtract, 33);
Set Deaths("Player 2", "Int:18535", Add, 1441792);
Set Deaths("Player 11", "Int:18535", Subtract, 369097852);
Set Deaths("Player 7", "Int:18536", Add, 25);
Set Deaths("Player 2", "Int:18535", Add, 50);
Set Deaths("Player 4", "Int:18365", Add, 50);
Set Deaths("Player 6", "Int:18538", Subtract, 3801088);
Set Deaths("Player 8", "Int:18368", Subtract, 3866624);
Set Deaths("Player 6", "Int:18535", Add, 7);
Set Deaths("Player 8", "Int:18536", Add, 402653184);
Set Deaths("Player 7", "Int:18535", Add, 14848);
Set Deaths("Player 9", "Int:18365", Add, 15104);
Set Deaths("Player 8", "Int:18365", Add, 7);
Set Deaths("Player 4", "Int:18535", Add, 4521984);
Set Deaths("Player 2", "Int:18588", Add, 32772);
Set Deaths("Player 3", "Int:18535", Add, 150994944);
Set Deaths("Player 3", "Int:18535", Subtract, 587202560);
Set Deaths("Player 3", "Int:18535", Subtract, 150994944);
Set Deaths("Player 3", "Int:18535", Add, 1140850688);
Set Deaths("Player 3", "Int:18535", Subtract, 1140850688);
Set Deaths("Player 3", "Int:18535", Add, 1610612736);
Set Deaths("Player 3", "Int:18535", Subtract, 1610612736);
Set Deaths("Player 3", "Int:18535", Add, 1375731712);
Set Deaths("Player 3", "Int:18535", Subtract, 1375731712);
Set Deaths("Player 3", "Int:18535", Add, 301989888);
Set Deaths("Player 7", "Int:17528", Set To, 3);
Set Deaths("Player 1", "Int:18593", Add, 32768);
Set Deaths("Player 11", "Int:17428", Set To, 10);
Set Deaths("Player 4", "Int:17429", Subtract, 234881024);
Set Deaths("Player 2", "Int:18590", Add, 32964);
Set Deaths("Player 7", "Int:18535", Add, 99);
Set Deaths("Player 8", "Int:17530", Set To, 3);
Set Deaths("Player 9", "Int:18365", Add, 99);
Set Deaths("Player 4", "Int:17428", Add, 5);
Set Deaths("Player 4", "Int:17428", Add, 512);
Set Deaths("Player 4", "Int:17428", Add, 786432);
Set Deaths("Player 10", "Int:18531", Subtract, 83886080);
Set Deaths("Player 6", "Int:18530", Add, 131072);
Set Deaths("Player 12", "Int:18530", Add, 65536);
Set Deaths("Player 11", "Int:27271", Add, 15000);
Set Deaths("Player 4", "Int:27287", Add, 15000);
Set Deaths("Player 3", "Int:18537", Subtract, 45);
Set Deaths("Player 5", "Int:18367", Subtract, 45);
Set Deaths("Player 3", "Int:18535", Add, 46);
As you can see, there's no magic here: regular EUDAs manipulating values. Apparently my program dump hasn't been configured to automatically convert EUDAs to their address values, but manual checks with EUDTrig suggest the majority of these are in the Unit Node table (just before the Unit Struct table in memory). It seems as though they're modifying the unit types to have different properties, as opposed to individual units.
There are a couple exceptional EUDs, though, near the bottom of the list, modifying addresses 0x653304 and 0x6535E8. The EUDDB doesn't have documentation for this area, but it's located somewhere after the trigger execution counter.
EUD Conditions can be used anytime, and EUD Actions can be used if you have an enabler.
What exactly are their limitations?
Short answer: EUD Conditions are limitless, and EUD Actions simply require an enabler.
Long answer: review Section 1 and Section 5 of
A Mapmaker's Guide for Creating EUDs to understand the usage and limitations of EUDs.
Are the Koreans ahead of us in EUD implementation?
Depends on what you mean by "implementation": as far as testing and pushing the limits on how we can use EUDs, we've always been the pioneers. But I know China, for instance, has more sophisticated tools for building EUD systems than we do, and Korea likely has more/better documentation oriented toward EUDs than we do.