I can't speak specifically about the Watt Bike, as I've not used it, but I do have the Predator Ergo and I do a ton of training on it.
October through March I do 5hrs a week on the Ergo- about 60% of my bike time.
April through September I do 2-3hrs a week on the ergo..
IMHO- it is possible to do equal or better quality Lactate training on the ergo Vs the Track.
That assumes that you can deal with Stationary training.. I can.
So that would cover accell style efforts- ridden from the Blue line,I do everything from 250m to 2Km
for those types of efforts you can basically replicate your track workout..
Ive gone through and done Test Efforts at the Track to determine effort time, average power and cadence for each effort, then I attempt to replicate the effort on the Ergo.
The weakness of Erg training would be replicating Speed Work.. Although flying 200 type efforts that I've done do have a similar power profile as on the track- but they don't feel the same..