glannin,
There are no major disadvantages. Lots of minor ones and lots of minor advantages too. Your bees will happily accept any comfortable home supplied for them!
Polys have been in use for 30 years or more. If there were major disadvantages they would have 'bitten the dust' long ago and never have caught on to any great extent.
Some parts of continental Europe are very much 'poly minded'. The North American Continent is not so well endowed with poly. The UK has retained a mainly 'wooden hive' mentality because, I believe, of Th*rne's pro-wood stance (they still do not supply any poly hives (only mini mating nucs)). I reckon that if it were to change, and they did start supplying them, the polyhive in the UK would become a great deal more common.
The arrival on the scene of M*dern Beekeeping had changed the picture only slightly due to the main polyhive, of Langstroth format, being a 'not so common choice' in the UK. The new poly based on the National format will, I believe, rapidly change the outlook of those producing only wooden hives.
Maybe not a good move for those making a 'class' niche product and perhaps it will not affect them so much, but for the mass-produced market, a polyhive should be very competitive. I can see the unit costs reducing even though oil prices are rising.
The one area where they will not score, in the short term, is for that of the self-build constructor because working with plastic is not so easy/convenient as working with wood.
One reason why most of my hives are/have been wood was my introduction to, and continued use of, the British National format and the unavailability of an alternative material of construction for it.
Regards, RAB