I view Delta cards as strong, of course for their initial bonus and for the benefits they convey to passengers (especially those without elite status) flying the airline semi-regularly. I do not view them as a strong tool for spending, independent of weakness in the SkyMiles program itself.
That’s because even if you want to earn SkyMiles through credit card spend, American Express offers cards that earn Membership Rewards points faster than Delta miles, and Membership Rewards points will continue to transfer to Delta as well as having the option of being moved to other programs as-needed.