Because the deposits accumulate, your grinder might simply stop working when it becomes clogged with sticky coffee residue. You can clean the reservoir often, but that won’t solve the problem of a clogged grinder. You can also send it off for cleaning every so often, but the oils will continue to build up in the burrs.
Eventually, your brew group could stop working after pulling too many badly brewed espressos. The oils will build up everywhere in the machine, including the tubing, which may eventually become fully clogged.
It’s harder to fine-tune the grind of dark-roast beans, because they react more strongly to grinder adjustments. This means the grind is a lot less precise, which ultimately affects the quality of the espresso being pulled.