The r29 preview removes the legacy android.toolchain.cmake path. Projects still setting ANDROID_NDK_TOOLCHAIN_VERSION will fail at configure time with a message that names the removed variable, which is better than the silent fallback older releases used.
LLVM 19 brings a new default for -ffp-contract on some targets. If your game relies on fused multiply-add for deterministic physics, pin the flag explicitly rather than inheriting the new default; we found two replay tests that diverged between r28 and r29 because of it.
The preview is not for shipping. We build one internal branch against it every week and report what breaks, which is how we found the floating-point change three weeks before the stable release. The full list of flags we pin is in the article on memory and in the build notes of our sample project.