I have heard that it is not advised to install a powered ventilator fan on your roof to reduce heat trapped in the attic, if you have a ridge vent / soffit vent ventilation system. Any inputs?

It is only unadvisable because it should not be necessary. Given the proper area of ridge vent and soffit vent you should have plenty of ventilation. I am sure you are not happy that is why you are interested in installing a powered vent. Check to ensure your soffit vents are not covered. Basic formula is attic Sq footage divided by 150. This will give total free vent area needed. You'll want 60% in soffits. Multiply free area by .6 then multiply by 144 for sq inches. A typical home depot 8x16 soffit vent is 56 sq inches net. 1000 sq ft attic/150 x .6 x 144 = 575.9 / 56 = 10.2 of those 8x16 soffit vents needed. 2 inch continuous vent gives about 9 sq inches net per foot - means you would need about 64 feet of that type of vent. Hope I helped and did not lose you.

Thanks for your response. It's not my house, but a neighbor who had asked my input. I'll take some measurements and do the math. There are full ventilated soffits, so there should be plenty of intake supply. Do you know anything about radiant barrier products?

Most people will find that there home does not have enough attic vents. When you do the calculations be sure to deduct for any restrictions like ridge mat and vent grills. The normal standard deduction for this is 25% of the total area of opening counted. A properly installed powered attic vent can save up to 1/2 ton of cooling but you also have to be sure that all the penetrations of the drywall in the ceiling next to the attic space is sealed or you can draw the cool air out of your home.