Is there any risk of the compile flag influencing which object owns the reference, and therefore causing a kind of "heisenbug" where it doesn't crash during the safe mode but still has dangling pointers in the fast mode?
Behavior will be the same in all three modes. There is however a chance that testing and development didn't cover a certain code path, and we would trigger unsafety in production, similar to unsafe blocks in Rust. When one encounters unsafety in Vale, they'll be able to just re-run in normal mode to instantly identify what caused it.
Normal Mode is very conservative (halts early when a constraint ref becomes dangling, rather than when it's dereferenced), so combined with test coverage, it can give high confidence in safety, and is strictly better than even C++ and ASan.
If that's not enough, Resilient Mode has zero unsafety, and with the optimizations we'll be using (nonatomic RC, Lobster's algorithm, immutable region borrowing, bump calling, etc) should be incredibly fast in practice, possibly on par with Rust and C++, and exceeding them in certain cases, with bump calling.