Not sure what Hackett means when he says "The hand/arm up to the underside of the armpit is not a part of the body that can score". Either he's referring to the rule as it was last year (in which case it's irrelevant) or he's correctly referring to the rule this year (in which case it was applied correctly on the Mane offside and is irrelevant to any complaints).
And if the cameras they use are 50fps then the PL really do have some explaining to do given higher frame rate cameras have been used for offside in other football competitions such as the women's world cup. They also use 320fps+ cameras in other sports. Hell, even the PL themselves use much higher frame rate cameras for goal-line tech as is. All provided by the same company, Hawk-Eye. So why they would randomly use much worse cameras for offsides, I have no idea. Seems unlikely to me.
Regardless though, it's the same camera used for everyone and even at 50fps would be a massive improvement in accuracy on judging by the naked eye. So if the alternative is to go back to having those same marginal calls made with less accuracy then that's hardly a recommendation, nor grounds for less subjectivity or bias (which the likes of Henderson seem to be alleging).