How reliable are relay modules?
Any module's reliability depends on the quality of the parts and the quality of the assembly and the quality of the design.
Cheap parts with wide tolerances put together without much thought into the design and assembled sloppily for pennies to make pennies by an unknown manufacturer and then shipped in questionable conditions will undoubtedly have a higher fail rate than a module made from quality parts, tight tolerances, quality design, in a high quality assurance factory targeting higher profit margins by a manufacturer that puts their name and money on the line with warranties and contracts.
Even worse if the random 3 dollar module you purchased is made with defective parts, factory seconds, ghost runs or straight counterfeits.