Yeah I noticed that as well... All the atmos units seemed to go off line after the fireball.
But as for the fireball itself, was it a mix of endoterm and exoterm chemical reactions the heat output might not have been that high. so the Valiant and the skyscraper would survive. As the Doctor wouldn't want to kill off humanity by using up all our oxygen I'm sure it wasn't a normal flame but rather a "flamelike visual reaction".
(And yes, even with no major heat the expansion of the atmosphere during the reaction would blow apart a vessel like the control-ship.)
As for Unit, nice to see them strike back with competence! But the best moment was when the Doctor crawled out of the teleporter, Martha cuddles up to him for comfort, while Donna gives him a slap for scaring everybody.
Nice representation of the two companions different personalities.