No, no data loss, but the risk is higher, because you need to rebuild which would have been unneeded if you had raid-class drives, since you would have never noticed there was bad sector on a drive and the raid-controller fixed it by writing the data somewhere else and flag the sector.
read this http://www.samsung.com/global/business/hdd/learningresource/whitepapers/LearningResource_CCTL.html
you will understand, desktop drives will work, just like in a pc, only raid-controllers are more sensitive. On a pc it might take a minute and you see your HD doing some weird stuff and the led is always on and then BOOM, error on screen that a certain file could not be read or something. a Raid-class drive will timeout and the raidcontroller takes over to get the good data from other drives. A desktop-class, like the WD15EADS, will be dropped...
QNAP could fix this, bit probably won't, by giving the user and option to set the timeout-time higher then 7secs. Some controllers allow it.
@ johannlo, all may work fine, your data isn't just as safe as it should be, meaning, you might see a drive being dropped quicker then it should caus of that.