I would add a buffer, maybe with discrete output transistors like a DOA but with a good audio opamp in front, at least a NE5534 but better is better. 50Ω/150Ω/600Ω outputs are easy to add, we don't use 50Ω a lot but is easy and may be useful for measuring transformers. AC couple is easy too, but would require a big cap to maintain really low output impedance and must be taken into account, that cap may be dominant as a THD source in some cases.
Using an output transformer for it at least as an option when using it for probing around without have to be careful with grounds is a good option too, probably I wouldn't use it for LF low THD stuff, and in any case we still will have the THD floor of the converters which unless you have a really good card could be a problem, and if you have that card you probably can afford a proper signal generator.
The THD of the DDS is almost 1%, you can't measure THD with it, I don't know the frequency linearity of it, I would trust my sound card much better, and if 1% THD isn't a problem or uncalibrated freq response isn't a hard job to get there with a DIY product, still can be a useful tool, if calibrated (at least you know the unlinearity) and the time base is good enough I can see a couple of uses for it.
JS