Amanos big tank uses them. I think the issue is the lack of stability, the lgiht fluctuates and algae gets a hold. I've seen a reef lit with daylight and nothing else but since finding it I've never been able to find the thread with the link in it.
Light tubes aren't favoured by many because what you gain in free light you then have to make up with for heating and you also have long tubes through otherwise usable space. They're also only a real option in bungalow or single floored homes as the practicalities of running the tube through a whole story sort of negate any solar gain.
If you go to garden centers and nurseries, you'll note they use shade cloth, so does Tropica and all aquatic plant growers.
This adjust the sunlight intensity, I use about 10% shade cloth for outdoor culture, this is about 150-200micromols at the top for where I live typically. Likely more seasonal variation in the UK for light.
So it can be used very well as long as you adjust the shading.
Frosted glass etc can be used also, metal screening in layers, inserts etc.
Adding metal screen inside the fixtures works well for hobbyists, for sunlight, the above suggestions work very well.