For me, it all comes down to the economics.
Avatars are an upgrade that costs soulstones, but since upgrades are also limited to three it costs one slot. In order to take that upgrade it has to be worth the actual cost of soulstones plus the opportunity cost of not being able to take something else.
Some masters, like Tara, only really have 2 upgrade spots due to how important some of there upgrades are. Therefore, those avatars better bring a lot of utility to justify taking up half of the remaining slots. Tara's latest one effectively competes with Dead of Winter, so it definitely is a useful option. I actually think Tara's is one of the best because it hits so many bases.
So, you have to keep the two costs in your head at all times. Second, as Justin said, some cards are strong to start, while others are strong only after manifesting. To me, it means for the same power of upgrade, you can play with the strength of the Manifest event and the SS cost to bring everything in line.
That is why I think it is possible for everyone to be happy overall.
Let me rephrase that:
That is why I think it is possible for everyone to be happy some of the time. We will never get all the people happy all the time.
Now my avatar Sonnia opinion: Why can't she just have the best Avatar? That model deserves it.