digital technology (such as the GFC700, DFC90 and DFC100) adds an entirely new layer of very good.
Yes, unfortunately mostly restricted to new airplanes. GFC700 and DFC100 are not available as a retrofit, and DFC90 is only certified for Cirrus and Piper Malibu and only for N-reg. S-TEC pretty much has a monopoly today for retrofits. Outdated technology at a premium price.
Once Aspen add altitude preselect to their PFD, they could provide everything a modern digital AP offers such as constant IAS climb/descent, a straight & level button, envelope protection, etc.