For SN1 reactions, secondary and primary carbocations are less stable and therefore less favored in SN1 reactions and SN2 reactions favor primary alkyl halides and methyl halides as electrophiles.
SN1 reactions favors tertiary alkyl halides as electrophiles. The Tertiary carbocations, formed by ionization of the tertiary alkyl halide, are relatively stable due to hyperconjugation with the neighboring carbon atoms.
Secondary and primary carbocations are less stable and therefore less favored in SN1 reactions.
The substrates SN2 reactions have less steric hindrance around the carbon bearing the leaving group, allowing for a backside attack by the nucleophile. Tertiary alkyl halides, with their bulky groups, hinder the approach of the nucleophile, making SN2 reactions slower or unlikely.