The pros to plugins and apps are they add some functionality
The con is you are adding queries, overhead, more ram usage, creating upgrade issues and slowing the site more with each one you add.
The key items to be aware of when using third party items, are as follows.
1. Do you have to have it? If not, less is more in terms of your time dealing with them.
2. It's a good idea to do some homework on the author of the item, as all third party items are supported by the author only.
3. Each time you upgrade, you first need to ensure that your third party items will work with the newest version you are upgrading too. (see #1)
4. How busy is your site? compared to how many items you are adding, there is a balance on performance when you are adding many third party items, as each one will likely increase the resources needed to maintain proper performance. It's a balance based on your site, your third party items, and how busy your site is.
5. A simple plugin can be fine on a normal traffic site, but put that on a site with 500-1500 online and it kills it, not many developers have a site with heavy traffic to test and optimize on, so this is a common issue on busy sites.
6. There really is no one size fits all answer to these questions, it would be based on many factors, your hosting performance, your traffic, and how many third party items you are using.
From a support perspective, when I see a site with 20+ plugins and 20+ apps, we tend to get to know those people very well, as they spend half there time chasing down issues with third party items.
My opinion, keep it clean, keep it fast, and keep it simple, more is not always better in most cases.