I'd like to add something to Raschi / Zauber(er) der Weihnacht as the reasoning for this idea is fairly complex to understand: Air units are in a very specific sense a balance nightmare.
If they get to powerfull they can easily destroy entire armies because only archers can attack them -so if they are too strong, they literally become uncounterable. However their stats are brought to the point where they may cause some damage this eventually overlaps with them being easily killed because while being only attackable by archers, they can't really run and hide.
While this problem could be solved if only few factions where in the game, it gets practically impossible, because of high the power level of archers is changing between each faction: You will e.g. never ever use Ringhunters against Lorien.
Also (because Edain revolves arround big armies and battles) there isn't really anything else air units can do than just attack people. Therefore at a certain point they utterly become useless because the ground armies (and with them the amount of archers) becomes too big to survive -still you can't buff them, because how uncounterable they are on the other side. This is an extremely fragile system which oscillates heavily between "overpowered" and "underpowered", and due to the amount of factions in the game, it is predetermined to stay this way.
Our idea now resolves arround the fact, that giving the Nazgûl secure, but limited attacks would essentially nullify the threat of being too strong (because you can't just spamm attacks into helpless ground units) while making it impossible to render Ringhunters useless if armies get too large (they have these guaranteed attacks even if they become more risky).
Also we brought up the idea of many different missions (e.g. attacking a single target or watching an area while being invulnerable and invisible). This is BTW perfectly in line with what the Nazgûl did in the WotR (book-version): They were given these missions (watching the Dead Marches, checking what happened with the Palantir in Isengard, terrorize MT during the siege and so on). It was also adapted into the movies where there are numerous shots of Nazgûl circling Barad-Dûr or watching Mordor.
Greetings
Melkor Bauglir