That might be the excuse they're using, but there's nothing that prevents it. It's important to recognize that when they're saying 'we won't do this because X' what they really mean is that they won't do it because it's politically bad for them and X is the reason they're doing it.
That being said, I think an independent, bipartisan congressional committee would be better than a special prosecutor. They would have a wider range of things they could look into and then you would give both parties subpoena power. As Nunes already showed, it's a bad idea to rely on the party of the president to subpoena testimony and documents if you're actually interested in investigating something.
I think we can all agree that the stench of corruption is so strong now that something has to be done, no? If Trump isn't guilty of anything then the whole country is better off having him cleared by an impartial investigation. If he is guilty of something then we need to get him out of office ASAP before he does any more damage.