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I really dislike the rental pokemons at S/M or USUM. It feels dispersonal because it is not your team of 6. Same deal with the talking rotomdex
Weaving HMs into your team kind of limit your options. You need physical flying pokemon for Fly, special water pokemon for surf and bunch of shitmons for Cut and Flash.
Having HM slave is the obvious option, but then it cuts your option from 6 to 5