No, it really is completely unproblematic; "But", you might ask, "what if a current leader does so as partisan retaliation, is not constrained by other forces, and starts of a chain of retaliation against past leaders?"
Sure, but using criminal process or any other power of office to punish partisan enemies is a problem whether or not the target is a past leader; there's no special danger around past leaders. if anything, there is less danger around past or current high government officials in general, and past Presidents in particular, because of the extraordinary degree to which those persons tend to be visible and well-connected politically.
Sure, but using criminal process or any other power of office to punish partisan enemies is a problem whether or not the target is a past leader; there's no special danger around past leaders. if anything, there is less danger around past or current high government officials in general, and past Presidents in particular, because of the extraordinary degree to which those persons tend to be visible and well-connected politically.