I would say it depends on which trainer you have; not only if it is zPower compatible but what peoples' experiences with it are. Even better, borrow a power meter for a day if you can and test your trainer for drift as it heats up.
I was using speed sensor only with a Cyclops Fluid2 trainer and it was challenging and frustrating. This was on Zwift as well as Trainerroad. I ended up getting a Stages power meter and immediately saw why. The issue was resistance drift as the trainer warmed up. As an example, here are two ramp tests, one with speed sensor and one with stages:
In the second one, you can see I held speed pretty much constant, but as the trainer heated up the resistance increased from ~100 watts to over 350 in 20 minutes of riding. Power starts to level out after a while of hard riding, but the drift makes virtual power difficult for trainerroad intervals, and infuriating for riding on Zwift!
Definitely not saying all trainers are like this, or even all Fluid2s are, but mine was not very good at all for virtual power, and it would be good to know how yours responds.