For instance, there may be several plug-ins that will try to shrink the image file sizes on your website, but if you enable all these features for the same purpose, wont the program behind the plug-ins conflict which with each other?
Another instance can be backing up your wordpress site. If the website has 2 back-up plug-ins and they run simultaneously for any reason will they not conflict with each other?
I have plug-ins installed due to recommendations of Wordpress/Theme such as Jetpack and Loginizer and both have "Brute force login protection" do they help each other or both just try to do the same thing and the other can be removed?
Since most plug-ins only offer "Activate" or "Deactivate" options, it is really difficult to tell if something may affect your site. And due to the complexity, it will be difficult to spot.
I have also seen several "guru's" tell people to install "this plug-in" and then "enable this feature" so it's constantly installing plug-ins while just enabling buttons without knowing what they do. So given thousands of plug-ins out there, it is overwhelming for beginners.
So what is a good way to determine whether the said feature is actually good/bad for your site?