Tried downshifting on a mile long hill (or so). The little engine revved up, disconcertedly so. The charging level wasn't that high either. I thought it would've used more regen breaking before adding on the engine braking, with a little more modulation of same. In years gone by I've been under the impression that brake pads are cheaper to replace than an engine.
I've found that using the CC to regulate speed works better and won't whine out your engine. If you need to brake, you'll need to re-engage the CC. If your really whining out your engine, your going way too fast to be in B mode. I only use B on steep down hills with hair pins to save my brakes. The decent down into South Lake Tahoe is a prime example of steep curvy down hill where you could smell the burning brake shoes of flat-landers. They're lucky that the stop lamp intersection at the bottom was replaced with a round-about, 5 years ago. There was always horrific accidents in that intersection, because people got their brakes too hot on the decent and couldn't stop. I actually saw tire skid marks going straight through up and over the round-about, the first year it was installed. Drivers licenses here seems to be a right, not a privilege.
The hill I tried it on was nothing like that. Next time I go that way, or another long hill, I'll try cruise.
It may be noisy and disconcerting, but isn't a problem, so don't worry about it. The ECUs control engine RPM and won't allow it to overspeed. And without any fuel, it is under less stress than when running at similar RPM to climb a hill. This is really meant for longer hills and vertical drops where regen will fill the battery well before the bottom of the hill, leaving you using brake pads long enough to overheat the brakes. There are some mountain roads where you can overheat and damage the brakes. That is why the Pikes Peak road has a mandatory brake check station, where an attendant uses an IR thermometer to check brake temperatures, and orders vehicles exceeding some limit to park in a cooling-off zone. We witnessed one while checking the adjacent gift shop, and it smelled horrid. A member here found another road, where he suffered heat damage twice before learning how to use B mode. For hills short or shallow enough that the battery will not fill up, you don't really need to use B at all. It is more energy efficient to use CC or the brake pedal to maximize regeneration instead. There is no "too fast to be in B mode." The ECUs set the engine RPM electrically, and won't allow it to become excessive. This isn't like a traditional transmission where a poor gear choice can mechanically force an engine overspeed condition.
You're misunderstanding what B mode is for. It's actually there to LIMIT regen so the battery doesn't overcharge on steep declines(think mountains, not hills). It uses the engine to bleed away energy instead of using the brakes. Even though the engine is revving, that doesn't necessarily mean combustion is going on. The engine is actually absorbing energy instead of creating it. Brake regen will still happen, but it will happen at a lower rate with a greater safety buffer so that it's less likely to get to max charge too quickly. It's only meant to be used during periods where you expect to constantly be on the brakes(say you're on the downward side of the Alleghanies or Appalachians).
Sorry; I'm not going to be the guinea pig to test that hypothesis; I pulse my brakes to control my speed and reduce engine revs. Unlike the flat-landers that ride their brakes all the way down the mountain.
