I can think of three types of lens reversal:
1) Reversing an enlarging lens: This swaps subject and image fields without changing focal length or working distance. The image field is usually flatter than the subject field, and we seem to get sharper results with such reversal.
2) Reversing a camera prime lens: This also swaps image and subject fields, but reduces the working distance to the lens register aka flange focal distance. For Pentax lenses, that is about 45.5mm. Reversal does not increase magnification, just shortens the working distance. For more magnification, add some extension.
3) Reversing a camera zoom lens: The field swap again, but the effect on working distance varies. From my experience with reversed 35-80mm and 28-90mm zooms, close-focus working distance ranges from about the register to several times the register, while far-focus at the long zoom end may be beyond infinity. A true macro-zoom!
With any of these reversals, aiming into a light tends to produce flare. Enlarger lenses typically aren't coated AFAIK and so may be most susceptible. But I avoid shooting directly into bright light and I haven't noticed glare with any reversal, not when shooting close. When I use a reversed zoom for non-macro shooting, I put a section of macro tube on the end to act as a lens hood. So far, so good.
We'd expect that newer digital-format zooms will have better coatings on rear elements. Unfortunately, these lack aperture rings, and so are more difficult to use reversed. Oh bother.