The answer is complicated because we don’t have a clear statement from google. A long time ago, Matt Cutts, the head of webspam team at Google mentioned that there should be a rel=”nofollow” in A tags of affiliate links because it was a paid link. Later on John Mueller said that Google had nothing against affiliate links.
Then, there is what the webmasters do in practice. The largest affiliate websites with high traffic all use either nofollow or a redirect with a “disallow” against a directory.
- thewirecutter.com (millions of sessions every month) uses redirects and disallow for every affiliate link
- nerdwallet.com uses nofollow on every affiliate link
- trustpilot.com uses nofollow on every affiliate link
The most successful affiliate websites use either a nofollow or a disallow. Some people mention that they have been penalized by Google because of affiliate links. Here is a story
So, it is not the affiliate link itself that can be bad for your website but perhaps the number of affiliate links in some circumstances.