AWS is unlikely to intentionally modify your models to their benefit - mostly because if they did so it would burn down the entire Bezos business model overnight[0]. However, any sort of donated computer time or blockchain that runs off completed training jobs can't rely on lower loss = model is trustworthy. It needs some kind of reproducibility requirement, which is much harder to meet[1] and is less efficient.
[0] It may also be illegal, though companies these days are very good at constructing the sorts of "opt out of the law" nonsense that sovereign citizen types could only dream of.
[1] Debian's ML guidelines defines two different levels of reproducibility. The most stringent one is that all the bits match; the less stringent one is that every weight matches to within a particularly low floating point tolerance. The reason why they have a second definition is that floating-point calculations generally do not reproduce across different hardware architectures. The exact same hardware will repro, but not between, say, an Intel CPU and Nvidia GPU; or an AMD x86 CPU and an Apple M1's ARM CPU.
[2] No, not that one.
Except that it's essentially a given that it will happen, because why would the NSA not demand this? The have access to everything they want access to, why would they not want access to something so impactful? Even if to only use that access very rarely.
Yes. Yes I have thought about this. No, I have not done it. ;-)
Frankly, the reward for this is typically very low compared to cost of being caught.