As I recall it, custom deliveries offer quite good EXP and scrips for the time invested into them. Tribal quests, at least in HW and beyond, also offer quite good EXP for the time invested into them, as well as tribe-specific currency.

Both are time-gated so that you can't use them to acquire an unlimited amount of EXP and scrips/currency in short order.

That said, I think that 3 per tribe per day, instead of 12 for all tribes per day, would be less restrictive and still preserve that time-gating.