Some comments from @sipa in #12702.

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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, ...).

{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            "loadwallet \"filename\"\n"
            "\nLoads a wallet from a wallet file or directory."
            "\nNote that all wallet command-line options used when starting bitcoind will be"
            "\napplied to the new wallet (eg -zapwallettxes, upgradewallet, rescan, etc).\n"
            "\nArguments:\n"
            "1. \"filename\"    (string, required) The wallet directory or .dat file.\n"
            "\nResult:\n"
            "{\n"
            "  \"name\" :    <wallet_name>,        (string) The wallet name if loaded successfully.\n"
            "  \"warning\" : <warning>,            (string) Warning message if wallet was not loaded cleanly.\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("loadwallet", "\"test.dat\"")