With no example scale to refer to, it guidance as you say, but it's extremely weak and subjective without a reference point to start from. It also creates a different scale by handling thresholds differently than in other parts of the book. Handling would be better expressed as a Threshold modifier and refer you back to the Threshold chart earlier in the book.
Let's assume the default handling of a standard vehicle should be a threshold of 3, since anything less is in the realm of walking and chewing gum, per the chart. That's your default for typical maneuvers. That still requires a baseline DP of 12 to execute, but as many have said, why are you rolling to turn corners? That's a Bad GM maneuver there.
So, any vehicle with a current Handling rating of 4 should have a +1 TH to the handling tests (4-3 = 1). Or you can set it to 4, and make the lower TH vehicles reduce the TH by the difference. A 3 Handling vehicle becomes -1 that way, and now you feel like you have that "better choice" car/bike whatever.
So, whipping around a corner at an unsafe speed, maybe drifting a little without a wipeout, sure stick with a 3+Car mod. Now you get crazy on the car though. You want to pull a Dukes of Hazzard maneuver putting the car up on two wheels. That's a threshold of at least 5 for sure. Now it really matters what car you have. No one is managing that junk without Edge and/or a lot of skill. Fine, yeah?
It should also matter that you have a vehicle rig. Right now, it adds its rating in dice, which is trivial by comparison to the requirements for most tests you are attempting, if the TH is going to be a baseline of 3. I would rather see it reduce people's thresholds overall, like it used to do. That feels worth replacing my spine for. Now my rigger can do stuff my wheelman can't. I can roll with the DP mods from speed better, and do notably nuttier things in that model. Right now, the rigger is only a slightly better driver than a wheelman.
I've seen the argument that the Edge generation from the rig enables the rigger to do stunts more often, sure. However you tinker Handling, there's that as a factor. However, outside of a rig, Edge has no rules for generation in a vehicle context. None. Which brings us back to the guidance topic: there's zero guidance for the GM on what stunts can even potentially do, or even the idea of doing stunts as a thing either. It leaves almost everything up to the GM, with no backup and no reference points. Very poor situation for new GMs.