> Heap flattening must maintain the integrity of objects. For example, the flattened data must be small enough to read and write atomically, or else it may become corrupted. On common platforms, "small enough" may mean as few as 64 bits, including the null flag. So while many small value classes can be flattened, classes that declare, say, 2 int fields or a double field, might have to be encoded as ordinary heap objects.
And maybe the end of the next paragraph is even more relevant:
> In the future, 128-bit flattened encodings should be possible on platforms that support atomic reads and writes of that size. And the Null-Restricted Value Types JEP will enable heap flattening for even larger value classes in use cases that are willing to opt out of atomicity guarantees.