I must say that I've been using DGDK extensively and I have had no problems whatsoever.
Of course I use native C/C++ code when possible, which is faster than DGDK functions, but this is the power of DGDK itself, to be able to integrate with C++ code.
This applies to the DGDK text printing functions, which I never use and are flawless in C/C++. It also applies to 2d functions like dbDot etc., which are sluggish.
Regarding the in-built bump mapping, it never looked very good. Using a custom shader is the way, it will obtain much more appealing results. You can find lots of these in the DBPro forum, and use and apply them in a couple of DGDK lines.
Other than that, my DGDK space sim has been tested for hours and hours and it has never shown any memory leak. It is solid and stable, and it uses all kinds of DGDK functions.