I think most of the timers are fair. Some of them are a little challenging, but nothing I can't figure out with a few tries.
The Level 3, Mission 7 is a little tricky though, I wasn't able to beat it without using a code to freeze the timer. Perhaps the timer could do with being a little bit longer, the main delay I had that was preventing me from beating the mission was the traffic vehicles getting in the way. I just couldn't get back to the C-Spanker from Kamp Krusty in time.