What is the downside to letting Gale reach his goals?

Finished my first play through and I encouraged Gale to take the crown, unlike Astarion who I diswayed from ascending.

I just didn't see the downside for letting Gale reach godhood. No one needed to be sacrificed and seems to be the only way to save Gale. Is there any reason not to let him take the crown?

Also how the hell do I save Karlach, she exploded at the ever end :(