The third guarantee of concrete is that it gets hard.
In regards to the 57s substrate, unless the 57s go all the way down to virgin soil, they will do little to prevent cracking or settlement. 57s on top of uncompacted backfill material still leaves you with uncompacted backfill material that will settle and leave voids under the concrete. The concrete then has three options--
1) bridge over the settled soil area (somewhat likely with 4' wide slabs if the outside edge sits on virgin soil, but more likely than with 3' wide slabs),
2) crack and settle, or
3) settle without cracking.
Reinforcement, such as welded wire mesh or rebar can help promote options 1 and 3, and discourage number 2. If the concrete does crack (which is most likely will, to some degree), rebar or mesh can help to keep the pieces on either side of the crack from sliding apart of settling differentially (one side settles more than the other, creating a trip hazard). Proper control joints can help discourage randomized cracking (they are called control joints in that they control where the concrete cracks).
In regards the strength of the concrete, unless you're planning on driving vehicles on your pool deck, the crushing strength of it is unlikely to come into play. The tensile strength is what drives its ability to avoid cracking due to inadequate support from below. Embedded fiber can help when there are tiny, localized voids under the concrete, but the settlement of backfill material will cause larger voids than fiber is designed to assist the concrete in bridging.
Uniform thickness is absolutely crucial. If you think of a graham cracker, it cracks easiest at the thin lines in it. Those lines are similar to the control joints that are either cast into, cut into or tooled into the concrete. Areas where the underlying material is not uniform in elevation prior to placement cause thin spots in the concrete which act the same way as an expansion joint, in that they are a weak spot in the concrete. Therefore it is unbelievably important to ensure that the substrate material is uniform in elevation in relation to what will be the surface profile of the concrete slab. 57s do provide an advantage in this area in that they're easy to rake and smooth out to ensure a uniform thickness of the slab.
Concrete of all thicknesses cracks and deteriorates, from 18 inch thick bridge approaches down to three inch thick pool decks. Proper preparation of the material below, particularly compaction of backfill, is the most important preventative measure.