Special Forces of all kinds have traditionally been more Force Multipliers than actual Main Force units.
So, as examples:
1. They all have Target Acquisition capabilities, so, in game terms, extend the line of sight ideally without letting the opponent know that he's been sighted.
2. Many have assassination functions similar to the current Spy Missions in Civ VI, but extended to 'hard' targets that they can engage by targeting them for heavy weapons (cruise missiles, drone-launched weapons, etc) or attacking with heavily-armed teams instead of individual pistol-armed spies. And their targets extend to not just Governors, but also Great Generals and politicians of all types (however the game wants to depict those)
3. Many also have 'Force Divider' instead of Force Multiplier functions: they can perform sabotage, especially of military targets like HQ, airfields, supply routes, etc. - places more heavily protected than the spy's 'civilian' targets. The original Soviet Spetsnaz were, in fact, almost entirely sabotage and demolition groups directed behind the enemy lines.
4. Most also have Deception functions - directing the enemy's attention away from critical points of the battlefield, distracting his command and intelligence apparatus.
In game terms, since they are almost entirely incapable of engaging main force units on the battlefield, Special Forces might better be modeled similar to Spies in Civ VI now: instead of units on the map, they'd be deployable to a point on the map (NOT necessarily a city) and either succeed or fail in their mission there and either survive to return for another mission or be destroyed.
Missions could include revealing all enemy units and installations in a given area (long range reconnaissance), destroying or damaging a military installation in an area (airfield, fort, shipyard), killing an enemy Great Person (primarily Great Generals, Great Admirals, but also potentially Great Engineers or Great Scientists) or even reducing the combat or movement factor of enemy units in an area (sabotage, demolition of routes) - but only temporarily: if you want to do permanent damage, use the Special Forces to reveal an enemy unit for the attentions of cruise missiles or Heavy Bombers.