How common does ed happen for those that take isotretinoin? does the benefit outweighs the risk of ed?

In 40 years of prescribing isotretinoin, I have never seen a single case of erectile dysfunction as a result of taking it. While there are rare reports in the literature of it happening, I would do a workup looking for other (more common) causes rather than automatically blame the isotretinoin. Even if it did cause temporary ED, it is a small price to pay if you have severe scarring cystic acne.