Conventional metal coated mirrors (typically aluminum) have been used in astronomy
for a long time. The advantage is definitely low price, meanwhile the disadvantage is
low quality. Reflectance is not high, ~ 90%. The serious problem is that environmental
durability is quite poor. Metallic film will deteriorate in a short time due to humidity and
cause the decreasing of reflection. But it is impossible to clean a surface, that is, not
possible to get the initial reflection any more.

Asahi produced the large 356mm x 229mm (diagonal: 423mm) all dielectric broad
band mirrors for the UH 2.2-m telescope in Hawaii in 2021.
Which will you choose, higher price but much improved reflectance, cleanable and
longer (almost permanent) life, or lower price but conventional reflection and much
shorter life?