I recently found one downside to Wordfence recently. On a website I manage, it used up enough resources that slowed down my website to a crawl even with cache option enabled. I was using a high-speed connection and yet it took about 2-3 minutes for the site to display.
The way this website is constructed is more to blame than Wordfence. In an effort to compartmentalize various aspects of the website, is made up of seven WordPress installs under the same domain. So in this case the site was using 7 times the resources a normal website might use. Wordfence simply pushed it over the limit.
The moral of the story most likely Wordfence won’t usually crash your website, but it may slow it down depending on the limits of your web sever.