smeezekitty :
That's not a sure thing. A backdoor can be written to be subtle and difficult to notice.
This logic is pretty suspect. SELinux closes many potential attack vectors, which are much more likely to be involved in any exploit against you than a hypothetical backdoor or two that it might open.
Now, as for the inconvenience and possible performance impact, I think those are valid reasons not to use it (I don't).
FWIW, I'm worried about NSA surveillance in general, but not of me, specifically. I worry about things like its potential misuse by unaccountable bureaucrats and political operatives. If Watergate happened in 2015, we'd probably never hear about it. That said, SELinux doesn't top my list of concerns and I feel it's probably no more exploitable than anything else Google might use.