I find TBT on Garmin is useful, but only as a convenience for beeping at junctions and making the screen more readable, when used with a lovingly hand-crafted (in Basecamp, using the map that's on the unit - anything else is too prone to randomness) Route. For anything audax-like, I'd use it in combination with a visible Track and the odd annotated waypoint so you can strategically ignore it when it loses the plot.
Ie. It's not a complete waste of time, but it does rely on you having done your homework, at which point its usefulness is greatly depreciated.
It's okay for helping you find the railway station when things go wrong, though. That's closer to what that sort of thing is designed to be used for.
I was reasonably impressed by the Edge Touring's "plot me a ride of distance n" feature. Again, a different animal from following some pre-determined route, which auto-routing is always going to be in conflict with.