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It's also difficult for both SW teams' and HW teams' visions to converge, even under the same company, such that the product can be put to use to maximize performance and programmability WRT another, already-established product.

Different constraints and challenges on both sides of the aisle give rise to compromises which end up with lowered performance or lowered ease of use. This is one area where great authority over the entire stack lends you lots of leeways, e.g. Apple designing Metal API and the HW for it.