1) That 25% damage on two girls includes herself, which is unfortunate. It's also in keeping with the total bonus amount found on most 5* units (ranging from 45-50%)
2) While you're correct that most gold "hit all enemies" skills are 1.7, AM(A) has two big problems: 1) an abysmal trigger chance (25% at Lv.5) coupled with 2) her ATK stat which is among the lower group of 5*. Increasing the multiplier to 2x only allows AM(A) to achieve parity with the upper tier of 5* units...if it weren't for that trigger chance. Compare AM(A) with Cymbidium:
a) Both have the same max ATK stat at Lv. 70 (and 2nd affection): 6270
b) Both have "hit all" skills; AM(A) with a 2x multiplier, Cymbidium with a 1.9x.
c) Both skills have a trigger chance of 20% at L.1, but Cymbidium's caps at 30% while AM(A) caps at 25%; at max skill level, Cymbi will outdamage AM(A) on average.
d) Improving AM(A)'s skill level requires using 6* blooms which are significantly more expensive than the 5* skillblooms for Cymbi.
No one is saying that AM(A) is an utterly worthless unit, but she's definitely not worth investing in. The question isn't even "do I have a lot of 6* units" but rather "do I have a lot of decent 5* units".