the two races are virtually identical, do not have the same ribosome.
Apparently it's not a question of race or species, it's a question of individuals. In the blood group issue, it doesn't help that we are all the same species here in mankind; individuals may still be incompatible with each other.
Now, nobody has ever even considered transfusing ribosomes in reality, because that makes no sense to current medicine: ribosomes are organs within our cells, whereas blood is something that either consists of cells or exists outside cells. It's currently practical to transfuse cells, but not to muck around with their innards.
By the time of TNG, though, doctors might have learned their way around these incompatibility issues by leaving the blood (or other organs) as is, and only transfusing key subcomponents of blood (or of other organs) that are indifferentiated enough that they don't have any rejection issues. The whole 24th century medical approach might be based on that - and even things that do have rejection issues, such as ribosomes (hypothetically, that is - nobody today knows whether ribosomes would be rejected), are handled according to this paradigm.
It's a more or less valid scifi analogue trick, having Crusher perform a procedure that resembles a blood transfusion, and then run into issues that resemble those present in blood transfusions. Worf may have been compatible, but that doesn't mean all Klingons would have been. None of the onboard Vulcans were compatible, but that doesn't mean 37% of Vulcans wouldn't be. And obviously Crusher thought that some humans would be compatible, too, because she also screened the entire human crew for potential donors.
And why can't they just synthesize his own blood? I thought with the 24th century's advanced technologies they would be able to do it.
The writer acknowledged that by inserting a line where Crusher declares the molecules in ribosomes "too complex to replicate". Which is probably for the best, because ribosomes are complex lifeforms on their own right; if replicating them were easy, then replicating humans should be easy as well.
Anyway, a nitpick here: Crusher wasn't really speaking of a
blood transfusion. She was speaking of a ribosome transfusion treatment of some sort, which may or may not have involved the patient's blood. Supposedy Crusher set out to repair every cell in the patient's body, not just his blood cells. Which is only logical, because blood cells in general don't have ribosomes...
Timo Saloniemi