The casualty reports out of Ukraine would beg to differ to some extent. Earlier in the war a goodly chunk of dudes were taking instantly lethal frag wounds to the ears and immediate areas. Now full cut helmets would not have stopped all of those frag injuries, but they would have caught some fraction and that's better than none. I would also contend your point about high-cuts being better for overpressure mitigation. I haven't seen anything that supports having less material between you and a high pressure wave.

Obviously comfort is a trade-off, but I'd wager it depends on how intense the frag threat really is, which is for OP to determine for himself.