Are the outdoor surge and transient voltages on the AC power line stronger?

Much stronger. Typical 20 mm diameter MOV (metal oxide varistor)-based surge protectors will fail with first strong nearby lightning strikes near the AC power lines. A combination of 40 mm MOV, gas tubes and inductor-capacitor filters must be used to divert the incoming surge voltage and current into the earth ground. Otherwise, failure of the outdoor UPS – and equipment powered by the outdoor UPS – will occur on a regular basis. Since outdoor UPS units are typically located at remote sites and high up on poles or towers, a bucket truck must be used access to the failed unit and perform the repair work, which is time consuming and expensive. Therefore, only a professionally designed outdoor UPS with outdoor-rated surge protection circuit should be used.