Shelf life baits are now falling into 2 categories; attractor baits and food source baits, where maybe 15-20 years ago most were just attractor baits.
As attractor baits, you don't want many around the hookbait, maybe 5 or so, with limited feed. You are relying on the attractor qualities of the bait for a pick-up. You could fish them (or a food bait boilie) over your background feed, groundbait, particles etc, but you don't want to prebait with them as the nutrition may not be high enough to get continued feeding.
A food source bait, whether shelf life, air-dried or frozen can be pre-baited, conditioning the carp into eating them permanently.
As a spotty youth, a bag of shelfies would last me weeks. I couldn't afford to pile in bait every weekend, so it was an out of the bag hookbait, with a stringer and maybe a few freebies catapulted around the area. Spods weren't invented, so any background feed was impossible!
The advance of shelf life food baits, like Trigga, Solar Seafood Takeaway, Crafty Catcher King Prawn and Crab and Sea Salt, Rod Hutchinsons KMG and Monster Crab means that now I'm comfortable with shelfies.
We then hit our next 'barrier', the type of water. A water where the carp are reliant on bait, of any type, 'overstocked' as it were, where the bait is needed to survive is a different kettle of fish (sic) to a water which is rich in natural food. The overstocked water you can get a take on anything, yet the rich water the carp may have a distinct preference for a good nutritious bait. They may pick up the attractor bait from being inquisitive but it may not last long as a viable bait. The food source will potentially be chosen over other foods; effort equals reward, or 1 food source boilie provides more nutrition than 2 mouthfuls of bloodworm, and is easier to eat.
We also have the DF spod mix approach. A background feed composed of groundbait, be that Vitalin, particles, pellets, and a boilie over the groundbait.
The boilie only approach, either food source or shelfie is now very rare.