Metal gutters have a low heat expansion/contraction coefficient. Additionally, they can expand or contract in relation to the weather conditions much better than plastic that tends to deform permanently with the strain of extreme weather changes.
Metal gutters take less damage as plastic ones due to ultraviolet light exposure. UV light causes plastic gutters to become brittle, weakening the brackets and rubber seals found at the joints. The result of such UV exposure and thermal movement is a parting of the joints and unavoidable leakages.