The first AC game I played was 2 a bit after it had came out but due to whatever reasons or circumstance at the time I only put a few hours into it and never really got hooked to the point of continuing. Fast forward about a year after Unity came out and I finally picked up a PS4 and was hunting for some cheap games. I see Unity for 15 bucks and immediately recall all the issues people were posting about around release but I figure what the hell it's only 15 bucks.
Turns out by that time the game had been patched to the point of being as it should have been at release. I completed my playthrough without seeing any major graphical or gameplay bugs and if I had any freezes or crashes they were at least few enough that I don't recall. In the end I quite enjoyed the game and was more than willing at that point to purchase the DLC to play some more.
With that said, I'm really liking everything I'm reading about the next installment, sounds rather ambitious and I hope they pull it off well.
Syndicate and Rogue have practically nothing and are both very worth playing. Unity and 4 have a fair bit. 1-3 have tons and it's painful.
Correct me if I'm wrong but Unity has no present time gameplay portions right? It has the video intro, if you can even call it that, and then occasionally the present time lady will talk to you and you'll go do a side mission or something. I admit it's been a while since my playthrough but I don't recall playing as any characters other than Arno.