I understand the basics of HBr/HI cleavage of ethers.

Question

Ether: R-O-R'

When both substituents are primary or secondary, cleavage occurs by SN2. Br- or I- attack less hindered substituent.

when one of the ether substituents is tertiary, benzyllic, or allylic, the ether cleaves by either E1 or SN1.

My question is: how do I know how the ether will cleave when there is a tertiary or benzyllic or allylic group? How do I know which side will be attacked and will it be E1 or SN1?

Thanks!