A sling is a tool, designed to do certain things well. There is no perfect sling that will do every lifting job well (if there was we would only need one type!). But for many riggers their "one" would be synthetic, which will outlift others pound-for-pound. However, in addition to the obvious melting and cutting because they are plastic, they have a shelf life.
All plastic starts deteriorating from the moment it is manufactured. This deterioration can be accelerated by exposure to ultraviolet (UV) rays (these come mostly from the sun but also from industrial gas lighting). In online discussions it has been raised as a big concern but it may only be a concern because we don’t know the rate of deterioration.
Slings, like most things in the crane industry, are rated to a fraction of their breaking strength. Slings have a 5:1 design factor, which means the rated capacity on the tag is 20% of the breaking strength. That is, the sling could deteriorate by 80% and still not fail as long as it was not overloaded. The problem is, we don’t know how much they have deteriorated.
The takeaway: Protect synthetic slings from the sun and other sources of UV rays, acids, and other caustic materials!