Some comments from @sipa in #12702.

___

Yes, I think `importmulti` is superior in every way, except:
* Slightly less convenient to use (it requires redundant information, as you have to explicitly state what you want to import, and then give the keys/... to enable that).
* ~Doesn't support P2SH-P2WPKH yet~. Added in #14454

The latter can be fixed by just adding support for that case. The former can be improved by adding 'default' cases (e.g. if you prove a WIF private key, it can permit omitting the address, ...).

{
    int64_t scanned_time = wallet.RescanFromTime(time_begin, reserver, update);
    if (wallet.IsAbortingRescan()) {
        throw JSONRPCError(RPC_MISC_ERROR, "Rescan aborted by user.");
    } else if (scanned_time > time_begin) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Rescan was unable to fully rescan the blockchain. Some transactions may be missing.");
    }
}
UniValue importprivkey(const JSONRPCRequest& request)
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 3)
        throw std::runtime_error(
            "importprivkey \"privkey\" ( \"label\" ) ( rescan )\n"
            "\nAdds a private key (as returned by dumpprivkey) to your wallet. Requires a new wallet backup.\n"
            "Hint: use importmulti to import more than one private key.\n"
            "\nArguments:\n"
            "1. \"privkey\"          (string, required) The private key (see dumpprivkey)\n"
            "2. \"label\"            (string, optional, default=\"\") An optional label\n"
            "3. rescan               (boolean, optional, default=true) Rescan the wallet for transactions\n"
            "\nNote: This call can take over an hour to complete if rescan is true, during that time, other rpc calls\n"