Off course it is hard to maintain vigilance for long hours, especially when nothing happens (out of the ordinary that is). But then again that's why some have what it takes to do this job and some don't. It's like excusing a waitress for being rude towards a customer because "it is hard being nice to people all day long". If you can't cope, do something else.
Understanding what is tough is one thing (makes for a good pilot), accepting it for a fact and then using it as an excuse for piss poor performance is another.
I make mistakes all the time but in my defense I seldom make them twice. And every time, every mistake, makes me pissed at myself because I should know better, be better. And as soon as I don't do my best at getting even better at what I do I might as well stop flying altogether since then I have become truly dangerous.
I've flown with colleagues, both left and right, that have had the attitude that "well it's hard so what can you expect - **** happens" and they are not fun to work with at best, although mostly much worse.