When installed on an inhabited structure, building code will require the correct cable and connectors. MC-4 connectors and photovoltaic wire are rated for ultraviolet exposure, fire resistance, and water resistance. 99% of any heatshrink available is not rated for at least one of those factors, and insurance would take a dim view of paying any claim caused by a failure to use properly rated materials. That said, there are cheap connectors and cables that are "rated" and of dubious quality. However MC-4 connectors are designed and rated for this specific purpose, finding high quality ones is the difficult part, the cable is easier (since it is less likely to be made in China).