2.4Ghz seems to solve everybody's problems. Except submariners.
However, there are still a few rumours around about 2.4Ghz issues. Spectrum have released a special marine version and implied that the air one was unsatisfactory for boats, but have not actually said why (AFAIK). This might be a marketing trick?
Some fliers have suggested that 2.4Ghz can suffer from occasional glitches as the propagation characteristics of 2.4Ghz can include aerial shadowing. Some 2.4Ghz systems have two aerials to minimise such problems, and Giant Cod has a multiplexer circuit project to allow two receivers to be dualed for the same reason.
I have not come across any definitive proof of these issues - but I have found that the very cheap 2.4Ghz sets can suffer from poor soldering and other quality defects. Their basic design seems good, but anything sold for these rock-bottom prices is likely to have some quality issues, even if it is just with the trim and the instruction booklet...