I said that all those demigods are more difficult to kill and a greater threat to your fortress than Unclean Beast. That's absolutely true. I would guess that you missed my point because you're the kind of player who only thinks about whether or not you can kill the enemy Demigod or not. That's not what I'm talking about. Many games come down to who is able to penetrate the other team's base properly and those demigods are better at it than the Unclean Beast.
And don't underestimate the ability of online players to cry over anything. Players cry a lot. Just because you consider UB superior to something else doesn't mean that's the consensus.
You cant kill UB if you are dead. Something being an unbelievable killer is basically the same as it being very difficult to kill. Yeah, if you have a situation where the DG is going to ignore you for some reason, Sedna might be harder to kill, but UB will always come for you and you will die much faster than it will, making it very difficult to kill. The UB is a MUCH greater threat to your citadel (I'm assuming that citadel == fortress in your paragraph above) simply because you cant defend your citadel if you are dead and citadels go down very quickly late game if they are left undefended by DGs. The only real way to threaten somebody's base late game as you say is to put pressure of death on enemy DGs. The late game UB is GREAT at doing that. QoT and Senda are not.
Late game is all about preventing enemy DGs from doing what they want to do - if you leave them free will they do catastrophic damage to your base very quickly. That almost always means killing them or threatening to kill them and thus driving them off. The UB is unbelievable at doing this late game, which is why it is a much greater threat to end the game than QoT or something silly like that.
In the midgame there is some value in simply being difficult to kill or drive off. Then you can sit and occupy a lane and rack up war score. Thats not nearly as important in the end game. Plus, with the kind of damage that an endgame UB, Reg, or even Oak can put out, nothing is really difficult to kill for long. A late game QoT doesnt stand up for very long at all to a late game Reg or UB - defensive builds always take a backseat very late game to the pure killers (with HP builds to make them a bit harder to kill) in my experience.