The thing is I genuinely think anything apart form diesel will cause long term issues on any diesel, even the tdis.
I have run my old cars on all sorts of old crap, and what you save up front tends to bite you in the arse when something expensive goes wrong.
I've been using biofuel for years. Like well over a decade. I've never run on veg, but rather run the transesterification reaction, with some amount of care.
I think at my peak driving, I was doing 20k a year in a 300tdi. I had a grand total of two bio related problems, maybe three -
1) It eats natural rubber. Which means the IP front seal in rotary Bosch pumps. A reseal with viton resolves it.
2) wax point is way higher than diesel. So this isn't a "it'll break it" thing but simply that it won't run below a certain temperature.
3) and this one is borderline - the wire to the sender arm (that's actually submerged in the fuel tank) failed a couple of times over a decade or so. I don't think it should have failed that often, so I'll blame the bio for that.
So, really, only the front seal and that's it. And once you change that one, it's then good for, well...I've only ever done it once.
I wouldn't run veg in any direct injection engine without preheating etc. although a friend has done untold miles in his PD VAG engine with twin tanking. He is sufficiently risk adverse that he wouldn't even put my bio in it as it was an "unknown" to him, and he didn't want to risk it. With that level of attention things do work, but with the slapdash way a lot of people do things, I don't think there's any hope for running long term in much without issue.
I'm not sure if I'm agreeing or disagreeing with you now! I think most of these things are possible but not when people take short cuts.
We have a Freelander 2 (that won't run on bio because of the dpf regen cycle), a 300 defender (just back on the road - I'll never put mineral diesel in it and expect no issues - IP is the same one as off my old disco that did all the miles and is resealed with viton), and a 101 with an international engines 2.8 - again I've had the IP setup with viton.