The main reason these wireless units are used is so the installers can save time in wiring up conventional ones and having to lift floorboards or chase into plasterwork, and if you do it in plastic trunking then the householders moan its looks messy.
the transmitter and reciever units do lose the signal between each other, the main reason being if you have a power cut or if the batteries go, you then have to re-establish the link which is usually easy enough if you have the instruction book.
honeywell wireless programable stats are worth a look as they uniquely operate at a higher frequency (around 860mhz) than other makes (around 450mhz) and are much less prone to interference as most other wireless appliances, mobiles etc also operate around the 450 mhz mark. also, the honeywell ones automatically sync with each other as soon as the power is turned on, they are known to be quite reliable and retail around the 80 quid mark.