Yeah, the only downside of heap storage is the alloc/dealloc cost -- once you have the memory, it's all just bytes in RAM, whether heap or stack.

It's not even a clear tradeoff for large arrays, because using value types on the stack implies lots of copies -- when you have a large block of data you want to refer to it by reference.

Also, stack size is finite and relatively small -- 2MB-ish default per thread on Linux, or is it 8MB? -- so placing large matrices there will work until it doesn't. Heap allocation has all of virtual memory at its disposal.