Downside of using an anchor line too thick is that in really deep water and big waves and with a strong tide running the amount of drag acting on the line doesent always let the boat ride the waves proper to the point that some boats can start getting swamped .
Best rope for anchoring or veering is nylon as is has a high loading will stretch and reduces snatching action .
Another good thing about nylon is that if anchored up and veering or backing up to something Say a sheer cliff or lee shore to pick up someone , using the high elasticity of nylon if your engine then cuts out or the prop catches a rock the stretch of the line (up to 50%before parting ) ) will pull you back out into deeper safer water .
Nylon is ideal as an anchor line or tow line for a boat but it's not suitable and can be dangerous in long lengths for pulling boats on slipways with a vehicle or tractor especially if the trailer wheels get stuck or bogged down and the wheels then suddenly break out
and yes I have seen a I ton boat suddenly fly up the slipway