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The one thing coating won’t withstand is thermal shock eg hot water. They’ll survive one or two dousing but sooner or later this will cause coatings to craze, quite like a crazed glaze on pottery.
The other risk is water in particular. No coating is perfect and all contain microscopic pinholes. Water being a polar molecule is very powerful at getting through these pinholes and in between the coating and the glass substrate. For many years you won’t notice but after 10-20 years you probably will.
The worst affected appear to be old MgF coatings which may peel prolifically - familiar with CRT computer monitors in the 1990s.
Eyepieces are easily replaced. But a scope like mine is not, so think carefully before applying anything.
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