When a board is retired, it may:
- get no support (never had support from Arduino zelf though)
- not be readily available.
Which may or may nog be a problem.
The biggest problemen with retired boards is that they often didn't sell very well.
So they may:
- target niche applications.
- not have a lot of people post tutorials on it.
- not have/get elaborate answers on Arduino.se
I believe you should only get something that fits your needs.
Usually you van get away with an Uno, and since there are millions if (beginner) tutorials, i'd pick that as my first Arduino.
If you want to do anything more "pc-like" like USB, camera or a "server", my first pick would be a raspberry pi.
Also, since there are hundreds of tutorials.
You can choose to do it with a yún, and it may work, but you should check if there are tutorials/documentation for whatever you'd like to do with it.