Optically, a light in a mirror is the same as a light directly shining on the subject. As Tetsujin says, they will however, have a higher effective distance. Whether this is an advantage or disadvantage depends on the situation and your taste.
Also repeating points in Tetsujin's answer, but (hopefully) expanding: the purpose of mirrors is to reflect at an exact angle, and create coherent images of the things reflected. You just want a reflection; for most purposes in photography, a coherent image of the light source is not needed and in fact a drawback. Since mirrors are more expensive than most reflectors, you're spending money preserving something (namely, light sources that are close to point-like) that you probably don't want to begin with.
Remember that all sources are reflectors, mirrors are just image-preserving ones. A white sheet acts like a mirror and a lamp shade at the same time. You do have to make sure the albedo is reasonably high, and you do have the inverse square law applied twice, but other than that, any surface can supply illumination. Also keep in mind that two surfaces that both look "white" can have slightly different spectra as far as what albedos they have at different frequencies, and your camera doesn't necessarily see color exactly the same as your eye does.
If you want more professional reflectors, you can do a web search of "photographer reflector". Basic ones will be