As far as I can see at this point, you can get a good feel by going to the Best Solutions for a Problem and clicking on the Python link up there to see how many people solved the problem. If there are a half-dozen solutions or more, then IME it is a good bet that a sensible algorithm that relies on built-ins will give you WA. On the other hand, if Numerix is the only person to solve the problem, then maybe not so much. (I got burned by this last week with TUTMRBL, which Numerix says in the problem comments is "easy" to get an AC in Python but I didn't find this to be the case and Numerix is the only person who has managed it so far.) But if you do your best optimization and still get TLE then I figure it's not a huge deal, since by that time you've got an algorithm that just has to be translated into C.
Speaking of CUBES specifically, I am one of the people who got AC in Python and my twelve-line script is pretty naive. The one mild spoiler that I'd throw out there is that checking for set membership is extremely fast, and a script that depends on that gives you the performance of C combined with the readability and rapid development that is awesome about Python. But, yeah, when I tried doing SQRT2 I also learned that my processor can run circles around the judge's.