engineers would be OP if there was no tradeoff with scientists.
That is a simple balancing issue. If the counters were split, obviously the costs would have to go up to compensate.
And I would actually like to see a little more balance between the great people types. Unless I'm just fundamentally wrong about good strategy in this game, Scientist specialists (+3 beakers) and Engineer specialists (+2 hammers) seem better than Merchant specialists (+2 gold), and Academies (+8 science? plus two techs that increase it by 2?) are much better than a Customs House (+4 gold), and bulbing for lots of science also seems better than a trade mission (it's also faster and easier).