Mirror lenses aren’t “bad” — they’re a trade-off. Their main advantage is that they can deliver long focal lengths in a much shorter, lighter package than a conventional telephoto. They also tend to have little or no chromatic aberration.

Their main limitations are:

• fixed aperture: mirror lenses generally can’t use an adjustable iris, so exposure is controlled with shutter speed and/or ISO (sometimes filters)
• usually slower than large conventional telephotos of the same focal length
• lower contrast in some situations
• distinctive donut-shaped out-of-focus highlights, which some people dislike
• typically prime lenses, not zooms

They’re useful when you want a compact, lightweight long lens and can live with the fixed aperture and unusual bokeh. They’re less ideal when you need maximum flexibility, higher contrast, or conventional background blur.

No: a mirror lens does not collect more light than another lens with the same effective aperture. The f-number already reflects the lens’s light-gathering ability at the sensor. In fact, the central obstruction in a mirror design is part of why these lenses have their characteristic rendering.