As others have said, the sensor that you can strap around the rear wheel hub is useful on a trainer or when using the rollers with a fork mount. Also, you can save some with some of the lower priced ANT+/BLE units that Amazon sells. Believe mine was around $22 and there were some under $20. It works flawless with the Wahoo fitness app and with the Cateye app (I don’t even have a Wahoo unit, just a Cateye Padrone Smart).

One interesting thing regarding GPS drift. If I have GPS on when riding the rollers, the map shows me wiggling up and down the street near my house, so that gives some idea of the inaccuracies when used to measure speed.