You can have high relations with another country and it will still declare war on you if your BB is bad enough. I just finished a game as Prussia, and I think you get about 40+ poitns for the unification event. If it's like EU2, then any score greater than 10 is not going to be good, and once you near 15 then you're likely to get BB wars; I'm not too sure of those values but a BB of about 15 is still fairly significant. I lowered it to 1 and then people left well enough alone for a while.
What I have noticed, in two games I've played (USA and Prussia) is that there hits a certain point when I stop getting inventions. I research things - like Infiltratin, for example - but then I don't get the invention trigger. It isn't listed when I highlight the research item. As the USA this happened fairly early; I never got Aeroplanes, electric gear, automobiles, etc; but that was 1.00. In this last game (1.02) then I got as far as getting aeroplanes and electric gear, but I never got some things like HQ attachments or tanks, despite having the requisite technology. :-/