I'm playing a romani campaign in 1.5 and I kicked the Epeirotes out of Taras within a couple of years of starting the game. About four years later I sent diplomats and spies around Illyria and into Greece and saw a couple of stacks sitting on the shores of Epeiros - I was still at war with them of course. Worried, I asked for a ceasefire with Epeiros and they immediately accepted - then I got trade rights the same turn. Sure enough, within a couple more turns, they had taken those stacks and attacked Pella.

I haven't seen anything at all yet that would make me be really afraid of the diplomacy in 1.5.