Mare Nostrum is the worst. Raiding is literally shit, as 1 click loots several provinces at once and 1 galley can make AI's raiding crippled(and if that 1 galley stops hunting pirates for some days, then all coastal provinces will be raided). Trade league is made for MR, but no one wants to play MR for estate-less, bad factions, and state province limit. Trade city is buggy, it makes me think that there is no QA in Paradox actually. Only condottieri is meaningful in Mare Nostrum.
Mandate of Heaven is worse too, as Mandate system and Shogun system can't give historical immersion and fun gameplay. Also, 350ish development of Japan compared to 1100ish Ming and 130 Korea is hilariously high and Korean communities went crazy after seeing that crazy development.