I can only speak for myself, but the main reason why I switched from BI to Alex is that the Qarthadastim was always, always, always going gung-ho after Tolosa as soon as I captured it, and especially so after I built mines there (or if they were there when I captured it). Nothing mattered, not even if we were allies and both fighting the Romani; they'd literally walk from Sicily through Italy, ignoring - and being ignored by - all Roman towns and armies that lay in the way just to get to it. As if that wasn't enough, they kept on sending fullstack after fullstack, one per turn, with a lot of those african elite phalangites, through their navy. This in every single campaign I started as the Arverni.
When I finally tried Alex, it loaded faster and that problem wasn't there; together with a more active eleutheroi, that clinched it for me.
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