I've never used a proprietary scanner I'm afraid, but I think one important issue, separate to the scanner, is that old slides are often warped - often not by much, but the focal plane of a scanner is tiny, and so even a millimetre of warp will make it blurry. Glass slides can help, but then you run risk of interference patterns developing if the warp refuses to squash flat against the glass. I've scanned some 35mm and 120 slides and negs on my flatbed with the provided mounts, and they weren't too bad, but negs tend to stay flat as they've never been cut and put in a mount. And I did scan them at something like 2400dpi, so it took like ten minutes to do one!
Another problem with old slides is that the grain structure was quite large and very soft, so they often need some processing and sharpening after scanning. Also they tend toward the blue/purple as they age, so colour-balancing will need to be used too.