Alkylation (R+ attack) — drawbacks:

1. Multiple alkylation: Product is more activated than reactant → polyalkylation occurs
2. Carbocation rearrangement: Primary carbocations rearrange to more stable secondary/tertiary
3. Does NOT work with vinyl/aryl halides (cannot form stable carbocations)

Acylation (RCO+ attack) — advantages:

1. No polyacylation: Product has -COR (meta director, deactivating) → stops at mono
2. No rearrangement: Acylium cation (RCO+) is resonance-stabilized, doesn't rearrange

Common limitations for BOTH:

• Do not work on strongly deactivated rings (-NO2, -CN, -COR, -COOH)
• Do not work on -NH2 group directly (Lewis acid complexes with lone pair: AlCl3 + NH2R → no catalysis)
• Workaround for aniline: Protect as amide (-NHCOR), do FC reaction, then deprotect