What do I know?
Actually, the Boeing 707 did not use bleed air directly for pressurization; it used turbo-compressors driven by bleed air but fed by ambient air. I was expecting someone to bring that up.
There's an interesting little experiment you can try using mashed potato. Dye some of it blue and then ask people to compare the taste of that with normal mashed potato. Most normal subjects will find the tinted stuff disgusting. Do that again in the dark: no difference detected.
That's a practical example of the "nocebo effect," the opposite of the "placebo effect." You can experience real, negative effects on flavor from something that works on your mind only because of what you have seen, and what you expect to taste because of that. The bad flavor is real, but it does not come from your taste buds.
In the same way, it may be that some people become ill from bleed air that is perfectly safe, because of that same nocebo effect.
I guarantee you that, sooner or later, someone is going to self-publish an account of how they crawled off a flight on a 787 feeling deathly ill because of contaminated air in the cabin. Not that you would ever want to do this, but I bet that if you went through the cabin of a 787 handing out a questionnaire about "Are you feeling anything wrong because of bleed air contamination?" you would get a few strong complaints.
That is not to dismiss the idea out of hand that some aircraft may cause real problems by supplying contaminated air, just to say that other things may come into play.