Use ～のみならず in written or formal spoken Japanese (reports, business email, presentations, negotiation) as the stiffer, more literary equivalent of ～だけでなく. It typically introduces an expansion of scope — from one party or factor to a wider one — and the second clause almost always carries も, まで, or さえ. Avoid it in casual conversation; ～だけじゃなくて is the spoken counterpart.