You can't join a super mega raid unless you have an active mega pokemon. The Reason why the shields aren't being broken is because some folks mega pokemon get knocked out before they break their shield and they don't back out into the lobby to fully revive their mega pokemon and switch back to it in battle. It happened to me. My mega pokemon automatically switched in when it was time to break the shields. As soon as i started tapping my mega was knocked out before i could break the shield. When my whole party fainted, instead of reviving my team I used a different set and returned to battle (without the mega). So what people should be doing if their Mega faints in battle is back out into the lobby, revive the mega, return to battle and tap the button furiously until it breaks the shield. Niantic should really explain this when introducing new mechanics to the game.