For the first couple of weeks they put out way too much UVB and this can burn retinas causing pain and possible blindness. After that, the UVB output drops down to almost nothing essentially leaving you with nothing more then regular lightbulb.
The idea behind using a fluorescent UVB light is to provide the UVB over a broad area so that it is absorbed throughout the day and not just while basking. MVB bulbs work as both a basking lamp and a UVB source but provide enough UVB while basking that no other source is needed. The coil type bulbs don't have a high enough output to work as a basking source for UVB and don't cover enough area to work like the full tube types. There has also been several reports of the coil types causing serious eye problems even resulting in permanent blindness.