I have problems with the Monica subplot in the final episodes not because it's implausible that her ability would have such limitations but because after DL's truly implausible death it looks very nasty to have her as the only character whose first attempt to use their powers deliberately leaves her threatened with a horrible death and weeping and begging for mercy and possibly getting another character killed. (The only precedent I can think of is Hiro getting beaten up by the gamblers in S1, but that's played more for laughs.)