Thanks for all the comments!

Since the drive appears to have been Voith, based on the first 2 Wiki articles below, perhaps some other references might also apply:

SP Krauss-Maffei locomotives: [en.wikipedia.org]'
SP Alco DH643 (minimal information): [en.wikipedia.org]

Both articles allege poor performance, especially in the mountains, and severe wheelslip issues.

Voith drive (appears to be a combination of torque converter, fluid coupling, and possibly hydraulic retarder): [en.wikipedia.org]
Voith Corp. - rail vehicles: [voith.com]
Recent/current Voith complete locomotives: Maxima [en.wikipedia.org] and Gravita [en.wikipedia.org]

So to summarize: they were an interesting experiment, but when traditional U.S. diesel-electrics became available with competitive power levels (c. 3000-3600 hp) in the mid-1960s there was no reason to try to perfect the oddballs. Based on what I saw in Wiki and some other references, it appears that the drives weren't really much of a problem. For some reason the locomotives were not capable of good operation in the mountains, but that may have been related to engine operation and cooling more than the drives themselves.

How different were they to operate, compared to "normal" diesel-electrics? Did they load as well at low speeds? With the fluid coupling, did the engines operated at near full speed under power (much like a modern CVT car), rather than notching up? Did they have any characteristic sound when the retarder (if equipped) was running, other than idling engines?