Up until MoP, I had a really difficult work schedule that did not allow me to fully dedicate to raiding. During Wrath, I was delegated to my guilds alt runs and constant pugging. It wasn't totally bad, I was able to play quite a bit during Wrath.
When i came back to the game during MoP, I was estatic for the new flex feature introduced in 5.4. As a player, I found it very difficult to raid because of the constant need for the 10 or 25 players to fill the raid.
My ICC experience in a nutshell: Fill 25 players for ICC, kill Saurfang, have somebody fly back to Dalaran to pug 6 people.
Flex in SoO was great. The difficulty was perfect for pug groups.
So in WoD, I decided to start a guild again and make it a purely Heroic-Flex guild. No more issues with filling spots. Just bring what we had and go for it. I was a GM/RL from WoD through ToS. Here is what I have concluded from flex raiding.
It was 80% blessing, 20% curse.
Like i said, it was mostly a blessing, but there was elements of the flex system that actually hampered my efforts to raid lead.
First and foremost is the healer/DPS ratio needed. Some fights on Heroic from WoD through Legion could allow you to overheal or underheal, while most required 1 healer per 5 players.
Secondly, the fact that we were completely flex meant that players ultimately did not feel the need to truly dedicate themselves to raiding. They would leave early when they felt like it, or not show up as often as they did before.
The 20% curse lied in the fact that your raid spot wasn't dependent on your dedication or ability to last throughout the night. You no longer had to play until the end of the raid time. People's raid spots weren't in jeopardy, ever. During Wrath, I was in a guild in which your raid spot would be lost if you missed one night because there were 5 people waiting for you to NOT show up.
The blessing was clear, we never had to shut down a raid night due to lack of people. In my 3 years as a GM (outside of holidays) we cancelled 3 raid nights total due to lack of attendance, and 2 of those instances we had 12-14 people, but only 1 healer.
While I seldom agree with him, Asmongold did make a good point on his stream the other day. He said in effect: "Before flex raiding, you had to bring in a bad player just to fill a spot, because that player was better than nothing."
What is your overall experience with flexible raiding on a guild level?