Do the mechanism (SN1 or SN2 ) which reaction follow depends more upon intermediate stability than on substrate and nucleophilic? For eg : would the reaction of 3-methylbut-2-en-1-ol with HBr follow SN1 mechanism due to stability of your intermediate carbocation stability  or SN2 due to the compound being primary alcohol and good nucleophilic. Also please correct me if I got something wrong.
It will follow SN1 as the intermediate carbocation formed is resonance stabilised.