I don't know that I would say that the RIAS is "less accurate" than the WISC, but it is shorter and doesn't assess skills in as many different domains. The domains that it does assess are highly correlated with "g". Someone who is not gifted is not going to get a high score on the RIAS.
Many WISC subtests are not even close to pure measures of intelligence, but are instead measures of intelligence plus motor skills, attention, or other functions. This means that the WISC scores (even the GAI, which includes both the block design and comprehension subtests) for some gifted people with motor or visual-motor disabilities, AS, or some other issues will be depressed. The RIAS is sometimes used as an entrance test for school gifted programs because it is both quick to administer and it isn't as likely to screen out children who have co-existing disabilities. The WISC is a much better tool to get a detailed picture of functioning and to help identify areas that might be in need of further evaluation to rule out disabilities, but it can be a "less accurate" measure of overall intelligence for those who do have them.
