rub
n. countablen. a problem or difficulty that is not obvious at first. You use this when a plan seems good but has one tricky part that makes it hard to do.
n. the central difficulty or problem in a situation. Often used in the phrase 'therein lies the rub' to identify a specific obstacle that prevents a solution.
The plan sounds great, but here is the rub: we have no money.
The new software is free for everyone to use, but the rub is that it requires a very expensive computer to run.
The proposal offers a comprehensive solution to urban congestion, yet the rub remains the lack of political will to implement the necessary tax increases.
Inherited from Middle English rubben, of unknown origin; possibly ultimately from Proto-Germanic rubbōną, related to reufaną (“to tear”). Cognate with Saterland Frisian rubje (“to rub, scrape”), German Low German rubben (“to rub”), Low German rubblig (“rough, uneven”), Dutch robben, rubben (“to rub smooth; scrape; scrub”), Danish rubbe (“to rub, scrub”), Icelandic and Norwegian rubba (“to scrape”). More at reave. Compare typologically Latin fricō < friō < Proto-Indo-European *bʰreyH- (whence also Russian брить (britʹ, “to shave”)).
Almost always used in the singular form, frequently preceded by 'the'.