There is nothing "unsound" about colour management in SilverFast and its conversion process from negative to positive is the most sophisticated apparatus on the market, but (i) you need to know its strengths and limitations and (ii) you need to know how to use it.
Let's start with the strengths and limitations. The main strength in this area is that it is the only scanner software on the market which provides negative to positive conversion algorithms which are CUSTOM-MADE for every film type, sub-type and ASA speed AND all these for every film-capable scanner for which a version of SilverFast is sold - a huge number of custom "profiles", considered important for optimal conversion quality because the tonal characteristics of each scanner and film type/sub-type/speed are different. It makes for a huge number of "profiles", but LaserSoft Imaging provides them. The main limitation of this approach is that all of these "profiles" were designed for the ARGB(98) colour space. This means that outcomes will likely be more accurate and predictable if you convert negatives to positives using the working space for which the "profiles" were designed - ARGB(98). However, one can very successfully convert many images using ProPhoto colour working space and the results will also be fine. However, for certain kinds of images, the remapping from an ARGB(98)-based "profile" to a ProPhoto RGB working space can produce some over-saturation. The way to avoid this from happening is to do the negative to positive conversion at the scan stage using a good Negafix "profile", and embedding (i.e. assigning) the ARGB(98) workspace (you do this in Options>CMS) to the image; then scan it and open the image in Photoshop; if you wish to work this image in ProPhoto thenceforward, convert the file to ProPhoto. The file numbers will change but the image appearance will remain identical.
Turning to knowing how to use it: the main thing here is to get the best Negafix "profile" fit to the film stock you are scanning. As I mentioned above, for every relevant scanner, the Negafix tool has a very large number of canned conversion "profiles" which you can test, obviously starting with the one made for the film you are using if you know for sure what film it is (often one doesn't, because of the age of the materials, re-branding of film, etc.); if the resulting display image is not correct as you perceive it, you can select the Negafix "profile" which has the best fit of all you tried, and then tweak it in great detail - especially if you use the "Expert" mode for the Negafix tool. Once you have the look you like for a normal. representative image, that tweaked profile can remain available for all other scans of the same film stock.
The fidelity of the resulting image appearance from SilverFast to Photoshop is very good.
In light of a number of comments in this thread, just to make sure I wasn't dreaming in SilverFastland, I fired-up my Nikon Super Coolscan 5000-ED and made several test scans of two image types (low saturation and higher saturation) and two working spaces for the latter, in order to see whether I could reproduce unsatisfactory results by doing things the right way; that didn't happen. All of it produced very satisfactory tonal and colour results, albeit, as one would expect, the appearance of a ProPhoto scan and an ARGB(98) scan of the same image with the same settings (apart from the colour space) is not the same. I did nothing to the files beyond the scan stage except to blank out the license number on an automobile for privacy reasons.
I have up-loaded to a server full TIFF scans as well as some JPG screen grabs comparing the results in SilverFast and Photoshop. These are large files, the TIFFS being 126~165 MB each and the JPEGs in the range of 2.6~4.7 MB each. One of the screen-grab comparisons is done at 100% magnification to show comparative detail retention for the saturated image in the two colour working spaces. It appears to me satisfactory in both, considering that this is a very large magnification of colour negative film which has not been treated for grain and not sharpened. For those interested, I am willing to provide download links allowing you to examine these files in detail on your own computers, but this will be only on request (contact me via PM from this Forum's messaging facility) with the provision of your full real name, your Forum screen name if different and valid private email address. The files are proprietary, copyrighted and provided for your personal inspection only. By downloading the files you are agreeing irrevocably to these conditions. Between the file names and the labeling on the images, along with the information given here, these images should be self-explanatory. Any questions or discussion about the images should be posted in this thread, as I am not likely to sustain a private email correspondence beyond providing the download link.