I haven't noticed a difficulty increase, as you say but I have noticed;
a)The AI takes notice to your dominance towards the end of the game. My last game as Venice (BETA Deity, Dom VC, Epic Speed) just today. I was friendly with everyone up aside from Bismarck and Atilla up until they all opted to go Freedom. I went on a capital-hunting spree, and after the 2nd they all decided to attack me (aside from Bismarck). I figured they knew I was too strong at this point and needed to all attack me.
b)The AI is quicker to be your friend. I drop a few free luxuries, and some 5gpt to neighbours and usually always get a DoF some turns later (usually on the 2nd give-away).
c)They're organizing plans in the WC better. The Venice game I spoke of, I fought back 2 votes for Freedom Ideology. The un-happiness would have crushed me since I was teetering back and forth, from red to green.
You'll get early wars if you're forward expanding, stealing with great generals, etc. Also a good way to provoke a war, so you don't take an ugly war monger penalty. Make sure to tell them to shove it, they'll be DoWing you in no time.