Quantum effects, particle physics, and then the unknown (for now).
The atoms below that are relatively straightforward, and them being made up of building blocks is fine I guess, increasingly irrelevant to the issue of a computer.
And going up everything starts to get very non-magical as you turn response curves into binary signals and then string gates together.
Btw I read your below word as above.
(And for difficulty making walls too thin lest electrons leak through, you don't need to invoke tunneling to explain that.)
Also I never understood things like slew rates, noise, gain bandwidth. Too high up the stack.