Spells aren't a problem at all. Due to the half HP, magic is more powerful, but that is a feature--not a bug.
It is also the reason why every creature has proficiency in all saves. For spells, such as fireball, which allow saves, it helps mitigate the damage a bit. Anyway, we've been using these rules in our online game for several months now without any issue so far. As far as something like magic missile, an orc in AD&D had only 5 HP so two missiles
would take the orc out. As we get to higher levels, things might unfold that need to be addressed, but right now we are just enjoying a faster, more pleasant game.
You mention the Ogre. In AD&D, the Ogre had 19 hp and your average fireball did 17.5 (5d6) at level 5 and more higher up. There is about a 40% chance the rolled damage will be 19 or greater, so on a failed save (65% IIRC) the Ogre is dead.
By comparison, with my system the Ogre has 29 hp and your average fireball does 28 (8d6) damage. Rolling enough damage to kill the Ogre is about 46% (close to AD&D 40%) and it against a DC 14 (+6 modifier; +3 proficiency, +3 INT) is the same save failed (65%).
It is nearly identical. I know it is just one example, but although there might be differences, the feel and impact is fine IME. Sure, changes might be in the future (I'd be surprised if there wasn't) but for now it works well.