Disadvantages of more than 8 speeds at the rear:

Tighter cogs - so less room for error when shifting. If using friction - harder to get it right by feel. If using indexed - cable friction, housing play, or any amount of RD misalignment causes poor shifting. 8 speed is more robust.

More expensive parts.


Advantages of more than 8 speeds:

Quicker rear shifting.

More tightly spaced gears.



As far as longevity goes, I have failed to measure any measurable difference in both chain, rear sprockets and front chainring life betweeen 8 and 10 speed stuff.