Extreme Encounters by Number of Enemies

So based on encounter design guidelines in GM Core, a combat encounter of 12- 16 PL-4 makes for a Severe to Extreme threat encounter. For those with greater experience or general game sense than me, does this hold true? Or do player characters get horribly overwhelmed by enemy action economy? Or do their crowd control and AoE options steamroll the horde of squishier enemies?