Be interesting to know why you think the previous model (9M sold) is unreliable,
100% reproducible fail for the compute heavy apps which it was tested with multiple Pi3Bs:
1. Running it as a time capsule - nfs mini-server exporting an attached 500G USB drive set-up for encrypted lvm. Fails ~ 10-20G transferred, classic symptoms of thermal failure. Going unreliable then random failure of running software, then hang or reboot.
2. Running it as a motion camera controller - 5 cameras, 1-4 H264 network and one USB. The moment the load is above >50% thermal runaway and thermal death.
Adding heatsinks delays the inevitable, but does not prevent it. Downclocking to any frequency above 600 only makes things worse because it spends more time at the higher frequency and heats up more. The threshold is ~ 50% at higher freq based on frequency stats. The moment you get into that territory you are a goner if you have something running on all cores - f.e. 4 motion threads. Same goes for making all cores busy with nfs+crypto+io.
The only thing the old Pi3B was excellent at was working at ridiculously low ambient. I had it running at -27C without any issues. +35C ambient was a definite belly up though.