in fairness those B&M dynamos are some of the better ones; typical bottle dynamos need at least 12W drag to get 3W out, those are perhaps 'only' 9-10W or so, (and have some means of voltage regulation IIRC). Contrast with hub generators which need 5-6W input to get ~3W out.
FWIW I think there is a difference in slippage tendency with bottle dynamos depending on what lamps you have attached to them. Halogen bulbs may be the worst; they have very low resistance when cold, so tend to draw a lot of current even at low speeds. This loads up the generator and makes it more likely to slip. By contrast LEDs don't really draw any current at the lowest speeds so a complete stall (skid) of the bottle generator I would think is less likely.
Maybe if bottle generators had been better BITD, I wouldn't have done what I did, which was to install a heavy (~900g) and low powered SA hub generator in my training bike in the early 1980s. I've done tens of thousands of miles (about a third of them in darkness) using that system. Apart from changing the bulbs when they start to look a bit too black, and the rivets that hold the hub together eventually working loose
, the system gave no trouble at all.
By contrast both battery lights and bottle dynamo lights were just a constant source of trouble and irritation. On the bottle dynamos there were wires that corroded or dropped off, junky brackets that slipped or broke or damaged the frame, problems of slippage, voltage regulation etc, all on top of the drag and the bloomin' noise of the thing. I could hear the dynamo very clearly wherever it was placed. I am sure that things may be better with more modern kit but that all the problems are solved I don't believe for a moment.
Modern hub dynamo systems are half the weight and three or four times brighter (at least) than the system I started with and are something of a no-brainer if you do a lot of night riding.