My wife and I have about 9 hours of testing at Velo Sports Center using ERO software and electronics to generate instantaneous CdA as we ride around the indoor track. Plus ERO does an in studio Retul set up where the position is compared to their data base of international racers and UCI pros i.e. what is your hip angle compared to X. Then one goes onto the track and tests different positions and using highly accurate equipment.
And ERO already knows what works and is fast so it is a matter of confirmation of the position and equipment on the rider.
Finally, one learns a lot from the experience about aero testing, position and equipment from ERO. Nothing beats an in person coach / tester with vast database and knowledge.
One thing about the indoor track testing and track testing in general is that it is head on. One is not tested at various yaw angles which is where the tunnel may be better. One of the BF racers, who has done both ERO and the tunnel prefers the indoor track.
One wants accurate speed and power reading and GPS will not work in the indoor track.
However, an on bike aero gizmo may offer some benefit just like workout apps offer some benefit to cyclists for training. And it may turn out to be great.