Jackson Emery made up for a lot of Jimmer's defensive shortcomings. But guys like Dairese Gary still gave him fits which is why BYU lost 4 out 5 to UNM Jimmer & Jax's Jr/Sr years with the only win coming in the game where Gary got injured.
BYU simply doesn't have a player athletic enough to stay in front of quick PGs with the ball in their hands, and haven't been able to do so for a long time.