To be specific and answer the question you posed though, the known differences are the following:
-ATTA is designed to work with any and all unit types. MELE is designed to work exclusively with melee units (as Zak explained, hand to hand combat units... basically excluding anything with ranged attacks)
-ATTA has many more specific parameters to make units do special things, such as flags like Attack Nearest, Attack Ground Location, or a Polygon in which the attackers will only attack inside of. The only unique parameter MELE has is a Power over Distance threshold (this will make your MELEing units attack a more powerful unit over a closer one when first targetting, or retargetting after killing its first target)
-MELE has the advantage of being able to specify a formation for the units to attack in, where with an ATTA you can't specify.
With the above in mind, I did a bit of research. The age old rule of "don't use the Melee action" seemed to ring a little foggy in my head so I went and asked Melekor what the deal is. He checked the lines of code relevant to the action and it's had two upgrades over the years:
// Build 225 - ALS - all games should use myth3's version of the code - the other version was clearly a bug (it didn't check bounds properly and produced crashes once target_index became too high and outside Myth's memory)
// Build 358 (1.7.2 beta) - GP ALS: Fixed a crash that could happen as a result of using too many monsters in a melee map action. (The limit is now 126.)
So with those two notes in mind, I'm thinking it's safe to say MELE actions are good to go, regardless of historical information about the action.
And as A-Red said, steer clear of MUNGER.... it's a very complex action and likely won't work for you as you expect, or will break your script and frustrate you. Only real time you need to start employing MUNG actions is when you've reached scripting limits and need to recycle actions you've already written. If that explanation makes no sense, it's a good indication you should stay away from Munging.
