That's completely false. They work just fine despite not being terribly efficient at least provided you install them correctly (but that caveat naturally applies to any window unit).

In fact despite the low efficiency using only one in a single room is likely far cheaper than cooling the entire house. It's the same principle as an electric space heater versus a whole home heat pump.

Of course running a minisplit only in the one room would be substantially better but for a 1 kW unit the difference is less than $1 per day (unless you're subject to the California electric grid I guess).