Its certainly beginning to look like a cultural thing as most of the complaints seem to apply to Gauls.

Now that I cme to think of it I was at war with the Gauls during the early game and never saw anything from them other than small wandering fleets. They sued for peace when I seized and enslaved their settlement at Salona and I haven't had any dealings with them since.

Perhaps its a problem with settlement size, Salona only had a tiny population and was quite incapable of being used to replenish losses amongst my troops.

Maybe they just have problems raising large armies.