At Eidolon 0, Phainon’s actions during his Ultimate are essentially locked based on Skill Points.
The general sequence looks like this:

E - QQE - QQE - Finish Ultimate

That’s a total of 4 actions. Assuming the Ultimate consumes around 160 Action Value,
this effectively translates to a Speed of 250 for the duration of the Ultimate.

For a character who benefits from Speed boosts (like action advance or push),
a 250 Speed off-field carry equates to about 125 Speed on a standard character (and that’s not even including extra turns from Bronya or similar supports).

Now think about this:
For a character with 125 Speed and no bonus damage on their Ultimate,
how much total multiplier per action would actually feel fair?

If you’re sensitive to numbers, you can probably draw your own conclusions.

Also, before Phainon activates his Ultimate, he contributes almost zero damage,
and each Ultimate round still costs a significant amount of action value.