Senate missions

As a Roman faction you constantly get mission froms the senate. I try to complete them as much as possible, even when they will only give you gratitute or a minor exotic unit. Is there a penalty on not completing missions? Or can I just ignore the less appealing ones?

Also, when I'm getting stronger I get the following prospect if I should fail:
"The senate will reveal your secret" and "The senate will investigate your actions" (or something like that). What do they mean? They sound threatening!

if they offer good units ( exotic units ) and so on you should do them, however never overextend yourself. It's just not worth it, and when you get really popular with the people the senate tends to get somewhat annoyed with you , no matter how well you do their missions, not always though I once took over Rome with my senate rating being 7 or smoething like that.
and when they threaten you you then should pay attention, most of the things they DO mean.those messages are the start of the bad things. if you keep ignoring them they will ( not immediatley ) outlaw you or fine you amounts up to 90000 denarii, which is very very annoying.