Set All Abilities To Level X

Is there a generic way to set all abilities of a unit to a certain level?
(without knowing what abilities the unit has)

No.

I found a simple workaround for this.
Upgrades with "Ability Level Bonus" effect.
And since you have 4 effects per upgrade it covers all I need for a certain unit types.

One upgrade per one unit type is better than making ability lists with triggers.
(4x better to be precise)

The bad thing is, once applied, there is no sane way of reverting the upgrades.

If you want "upgraded" abilities (the level will be the same, but the ability will be changed to whichever you had chosen), you should use the Engineering Upgrade. But I don't think that's what you're looking for.

I never understood why people complain about that.
Reverting upgrades is needed only if you want to have multiple replays in your game and in my experience that is not needed because in 99% cases people leave after first play.
(Even in established games like ENFO)

Oh, but there is much to complain about.

Imagine that the effect that you're trying to achieve with an upgrade is associated with an item.

Now, when the item is picked up, you're supposed provide the effect, and when it is dropped, you are supposed to revert it -- but you can't. And what are you gonna do now? There's only one choice - make the item undroppable, which, most of the times, is rather stupid. There are several other cases as well...

Well, I've spent some time playing Castle Fight in recent months and in that map at least, people stay for multiple rounds. Then again, there is a game message at the start of the game telling the players not to leave after the first round.

Exception that proves the rule.
The fact is that in most types of maps you do not need rematch capability.

I agree. I see it mainly as a matter of preference. I, for example, would have rather specified abilities on a per unit(type) basis even in situations where the limitations of an upgrade are not an issue.