I would like to get wider angle photos than my current setup allows (18mm APS-C minimum), but decent wide angle lenses (10-14mm for Sony) cost 300+€. I stumbled upon these wide angle converters which promise wider angles at much less cost and are screwed on top of the actual lens. What are the disadvantages of using those?
Everything is a trade-off. (i.e., "Good, fast, or cheap; choose any two.")
Convertors are cheap, but they pretty much all introduce hard to control chromatic aberration and can reduce sharpness. They also reduce the effective amount of light entering the lens. The former can be fun for artistic purposes, or if you can balance the chroma problems with a deft hand in post-production. But the combo of lens and convertor will never be a super-wide, super-fast lens. Loss of sharpness can never really be corrected for with current equipment.
As some wise folks have said, if convertors were as good as a wide-angle lens, no one would buy wide-angle lenses.
(It's true that wide-angle fans should consider sensor size first if wide-angle is a primary concern. Though, that being said, the APS-C ready true wide-angle primes made by the top manufacturers are so awesome these days. Even if you can find an actual deal on eBay for older super-wide primes for your smaller sensor camera, the newer glass designed for smaller sensors is often so much better.)