This could easily be dealt with by asserting the rareness of Occlumency in the HP universe, rather than making it a common skill among pretty much every adult wizard as fanon tends to do.

 

Or you could say that only the most stubborn of the stubborn are capable of resisting it, and that even then there are some wizards who are much more capable with the curse than others (Mulciber comes to mind). Fact is that most people don't have that level of willpower, so likely as not there is no 'learning to resist it', only learning the feeling to look out for.

 

Just because there are certain measures against it doesn't mean that Veritaserum and the Unforgivables are weaksauce - hardly any people can resist them, and apparently, in the case of Veritaserum, with preparation.

Making it infallible is very much Deus Ex. We need to know whether Snape is a spy? Serum, bam, we have the truth. No spies would exist, no subterfuge, no guessing someones allegiance. Boring.