Am I understanding this correctly: a value type really only works when it fits on a 64 bit "cache line", and when larger, it falls back to normal heap allocated objects as before? Seems extremely limiting, no? Great for a boxing optimization, but not much else unless you're deal with very small data types regularly...

That's true for arrays of these value classes. Scalarization would help for larger local values though, since those would avoid pointer indirection for purely local values.

And one bit for null! So “smaller data types” means 32bits! Yay! It’s 1995 again!