If Diana Ross and Gladys are from the Motown Hit factory, than you can also consider that Dionne Warwick is from the Bacharach/Davis factory.
If Dionne had different style in each decades, Diana is the same [[I don't know for Gladys).
Are Dionne songs so difficult to sing? Maybe but many singers covered these songs very well and with success.
I like Dionne, mostly for her Bacharach catalogue but she is one of the most boring performer I'v ever seen.