When my fluid trainer heats up the Zwift power curve becomes 25%+ innacurate (Elite Fluid Ritmo - Elite Red roller). I think this is an issue for all fluid trainers where the viscosity of the fluid increases with temperature and therefore the resistance goes down.
Simultaneously looking at power from my power meter pedals (recording on my own account) and from a speed sensor/zwift power (recording on a trial account) revealed that power matched for the first 5 minutes, but after 15 minutes (of 250-300w) it was innacurate by >25%. Pushing 300w on the power pedals was recorded as 400w by zwift power. The faster I go, the bigger the % inacuarcy.
Fluid trainers should not be used for training plans or Zwift races