I wouldn’t let this discourge you too much, I pug alot and for the most part it goes well. I wouldn’t focus too much on what others did wrong, but how you can improve to avoid disaster.
I found that an addon called " Premade Group Finder" and a WA  “Dungeon RIO and Class” goes along way.
I have found that joining a group that already has a tank and healer often sorts most of the problems out, as they set the tone of the group dynamic, i preffer a tank or healer with a BREZ as that is utlitiy a mage does not bring. I also wouldnt suggest a double mage group ( unless they are friends) as you will run into kick issues and quaking doesn’t help casters. You can also filter out groups that already have a mage.
A tank pulling singles packs isnt a problem at a 14 level at the end of a patch, if your
team is having problems its likely due to the kicks or the affix. One person not being optimized shouldn’t be much of an issue aswell, I was doing 14’s well before i had a leggo and I can can say, when farming valor or renown, i have at times, brought the wrong leggo and still creamed the key.
I believe this is also why they added the timed vs completed goal option. Expect leavers in a timed advertised group if things dont pan out.
Hope this helps, Success is not final, failure is not fatal: it is the courage to continue that counts.