>I'm starting to think that it's impossible to get an accurate blood work from a finger prick only.
It is possible, but just for certain test types and error rates. The basic problem is the blood drawn from a finger prick is affected by exactly where it goes in and how much skin tissue is picked up. This variability gets averaged out with a large venous draw and so is not quite as bad - there is still a large variability with even normal blood tests for certain test types.
The tests that work OK are things where you just want a yes/no answer (do you have HSV1) and very poorly for things where you are wanting to measuring exact levels of something.
None of this is new to the industry.