GrapeheneOS are both operating systems where a single party has the signing keys to run any software they want on your device at any time, and even remotely brick it.
I for one am not comfortable with anyone having that much power over something that is supposed to be mine, so they are the same to me. Untrustable black boxes.
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The difference between GrapheneOS and iOS is that it is 100% open source, replicable, able to self-sign, and only <1% of the part has dynamic code loading, which is incredibly low compared to any major OS.