Well i would say the biggest change is oak, when he hits level 15, he gets much more powerful compared to mid and early game Oak because of all the reasons mentioned above.
Ub and Ere are very good late game as well, however the change in level 15 is not as drastic as Oak.
Ice Tb becomes much more useful with long skill interrupts and 3 sec freeze. So i think even though he is still weaker than the other 3 demis, he has drastic change.
A speed fire Tb is good for capping flags and runing around to keep the troops away. But i think only speed fire is useful for late game.
Sedna is good for 2w2s to cap but on a 3w3 match it would be harder for sedna to cap dap. Since it is very hard to kill sedna with only 2 demis late game, but on the other hand she cant spike no damage on 3w3 and she is killable.
Reg is at his best during mid game while he is not as useful at late game. Mid game his slowing mines and betrayer spikes aoe damage better than any dg including tb. Espcially regelus with staff of renawel can do wonders in mid game. However he lack escape mechanism on group battles, so he is not very preferable for high level games. 1on1 encounters he can always get away thanks to mark of betrayer.
Rook is one of the best demis late game too. His stun time increases, he gets 1.5k dmg on hammer and slow, and posion arrows on top of that. he can tank quite a lot of damage. Rook has the highest Dps after Ub late game and towers are great for teleporting espcially late game. So i think rook has one of the best level 15 skills along with oaks skills and Ub's acclimation and Ere's mass charm. I take rook as the best baiting demigod, which is very important for high level games. It is usually very unlikely that someone would get a kill without a succesful baiting.
Qot is just unfortunate about level 15 skills. She is in her prime time in level 7 and 10. Then apart from the last rank mulch, she doesnt get anything useful and mulch is not as useful in 3w3 games.