Do you have a specific example?

Think about the resonance forms of the conjugate base.

Substitution reactions to install hydroxy groups are very difficult. In theory you could nitrate, reduce to NH2, convert to diazonium, and then KOH, but in practice, you could start with benzaldehyde and install a temporary directing group for directed ortho lithiation followed by an oxygen electrophile

Yes, but it would be difficult to conduct an EAS reaction in the presence of the double bond, which tends to be a better nucleophile.

Let’s say you did 4 different electrophilic aromatic substitution reactions (e.g. nitration) with PhCl, PhNO2, PhCF3, and PhCN. You measure the rates. All all slower than benzene. Of the four, one is fastest and one is slowest. You’d probably call the one that is the fastest, “weakly deactivating” and the one that is the slowest, “strongly deactivating”, and the ones in the middle, “moderately deactivating”.