That does mean there are certain assurances of neutrality we simply can't have. It doesn't mean that efficacy and safety can't be studied: they can.
Also, dosing matters. Can the apparent positive effects occur at low doses where possibly it isn’t very perceptible? This is important and a double blind approach is warranted.
I don't think it's as important for the attending scientists, who are also going to have a pretty accurate guess. What's more important is that they have some training in 'trip sitting' and that the experimental environment be appropriately designed, DMT: the Spirit Molecule has a number of stories about how things go poorly when this aspect is neglected.
Doesn't help: any normal person can tell the difference between a full dose of psilocybin and a low dose of amphetamines.
"You'll be given either a psychedelic or a stimulent, followed by x testing" - then knowing which drug you're getting doesn't mean you know if you're in the placebo group or not?