Are Benzaldehydes less reactive than aldehydes due to the -R Effect of the -CHO group present on the benzene ring?
For a carbonyl group to show reactivity toward nucleophilic reagents there should be positive charge on the carbon of the carbonyl group. The existence of positive charge on the carbon allows the reagent to donate electrons much easily.
Alternatively, A compound with more stabilized positive charge will show less reactivity towards nucleophilic reagents. The positive charge could either be stabilized by resonance or electron donating groups. In case of benzaldehyde, the positive charge is highly stabilized (due to resonance) than in other simpler aliphatic aldehydes.