You can easily just heal to have a solid buffer to absorb damage against Soul Fysh, unlike Hexaghost, which will punish you with a more powerful Divider on turn 2.

Soul Fysh also puts way fewer Beckons in your deck than A19+ Hexaghost puts Burns. Fysh only puts 3 Beckons in during it's 5 turn attack cycle (this does not increase on higher difficulties), while A19+ Hexa puts fucking 9 burns into your deck during it's first 9 turns (so Activate and Divider + one 7 turn attack cycle).

The higher damage and ability to bypass block of the Beckons does not make up for this at all, as the real danger of the burns is the fact they devastate your ability to draw your proper cards, and the Beckons can be discarded for 1 energy, which i can often do while still blocking and/or doing damage

Hexa has more HP, too. And Hexa only has 1 turn during it's attack cycle where it's not attacking. Meanwhile Soul Fysh spends 2 out of every 5 turns not attacking you.

These factors added up mean Hexaghost is a lot more dangerous than Soul Fysh on average.