Because SO strongly suggested to continue in a separate thread, I start it here (I hope it will be O.K.).
A question to TB: which units are supposed to merge/split and which do not? In the older versions of C2C almost all units could do that, including workers and Great Hunter units. Now this is reduced.
Any units that have property controls should not be able to merge or split. For game balance with this in mind, and in consideration of if they can as effectively perform their 'role' if they were a larger group, Explorers cannot merge or split either. There's some other considerations as well, where game effects aren't capable of being made larger with larger size. Workers were added to the list recently due to both being able to include Ecologists and Park Rangers (property control units) and some bugs and difficult to mitigate complexities with workers that sacrifice themselves when they complete their builds.
This means the list of UnitCombats that cannot merge or split will be:
UNITCOMBAT_BIOLOGICAL (Biological Missiles)
UNITCOMBAT_SPY (Spies)
UNITCOMBAT_MISSILE (Generic Missiles)
UNITCOMBAT_WORKER (Workers)
UNITCOMBAT_SETTLER (Settlers)
UNITCOMBAT_DOOM (Doomsday Missiles)
UNITCOMBAT_MISSIONARY (Missionaries)
UNITCOMBAT_COMMANDER (Great Generals turned Field Commanders)
UNITCOMBAT_TRADE (Merchants)
UNITCOMBAT_HERO (Heroes)
UNITCOMBAT_DIPLOMAT (Diplomats)
UNITCOMBAT_COMBAT_WORKER (Combat Workers - not in game yet)
UNITCOMBAT_CRIMINAL (Criminals)
UNITCOMBAT_EXECUTIVE (Corporate Executives)
UNITCOMBAT_SEA_WORKER (Sea Workers)
UNITCOMBAT_EXPLORER (Recons and Hunters)
UNITCOMBAT_LAW_ENFORCEMENT (Law Enforcement)
UNITCOMBAT_RUFFIAN (Ruffians) - Note this wasn't included as it should've been and you've found a bug to say that they were able to merge/split. I overlooked this adjustment in the review - to compensate for their inability to merge or split in the face of the fact that they should have very strong military value, they have been given the ability to start off a cat higher than norm, making them the only unit line that does this. So they will seem quite strong but military units can still easily merge past them.
UNITCOMBAT_STRIKE_TEAM (Strike Teams)
UNITCOMBAT_ENTERTAINER (Entertainers)
UNITCOMBAT_ADMINISTRATOR (Administrators - lawyers and judges)
UNITCOMBAT_PRODIGY (Great People)
UNITCOMBAT_WILD (Wild Animals - this may change soon.)
UNITCOMBAT_SUBDUED (Subdued Animals)
UNITCOMBAT_IDEA (Tales of)
UNITCOMBAT_BALLISTIC (Another Missile category)
UNITCOMBAT_CAPTAIN (Another Leader category)
UNITCOMBAT_ATTACHE (Leader type units that attach themselves to other units)
UNITCOMBAT_CAPTIVE (Captives)
UNITCOMBAT_HEALTH_CARE (Healers - though I think this list may need to expand since there's probably still some healer units that don't have this designation.)
I will check if City Guards may do that (Edit: no, they cannot) - the situation I had described was about these buggy City Guards and it could be caused by loading a save from earlier SVN version (I was unable to split them back).
This brings up an interesting complexity. Some non-LE units can promote to LE units which can mean larger grouped LE units. This strongly suggests I may have to do something to enforce that they break down or merge up to the original size to be valid for upgrade to a unit that cannot merge or split. LE units haven't been able to merge or split for a long time so I'm very curious what previous unit type they were at some point that allowed them to merge.
As for Hidden Nationality units: IIRC Ambushers, Asassins, Hashashins and so on are unable to merge/split, unlike Bandit Footpads and Riders.
Again, thanks for pointing out the bug with Ruffians. Criminals and Strike Teams cannot merge or split. Criminals because they are an inverse property control unit with multiple means of sacrificing themselves for missions (which is usually a reason to designate inability to merge or split so as to avoid enabling gaming the sacrifice.)
Strike Teams cannot as a balance factor. They can get so very strong that SM helps to keep them in check by enforcing them into a standard size unit group (by standard I mean that when trained their strength is as it would be in the core). This makes them very useful at picking off weaker targets while making it difficult for them to take on the big army size units. It also enforces that they use quality to upgrade their SM values, which if they are used very strategically can easily earn their way up to.
Ruffians are more of a swarming unit of less disciplined, but somewhat stealthy types so they start off at a volume shift higher than core game strength but being limited to that makes them perfect to be a 'thorn' strategy unit that can present a threat to an opponent nation that must be handled with brute force. They can hold their own but ultimately are limited and enemy units that face them may be merged even larger since they cannot further merge themselves.
Law Enforcement units may frustrate some who like to use them as primary defenders. This currently is leaving cities without strongly valid melee city defenders and next version will address that. Since properties cannot be adjusted by merges and splits, this is flat necessary but also helps to keep them balanced against criminal units.