Addendum

In the meantime I have tested Decklink with a computer based on the latest Intel motherboard, with P4 3gHz with 2mb L2 cashe, 2gb ram, Nvidia Quadro PCI Express graphic card, separate 200 gb disk for audio and 600 gb SATA stripe for video.
The performance changes a lot and suddenly everything works smoothly even with higher video data rate. Nevertheless, when computer load was close to 70% the video becomes jumpy. But to keep things in perspective, video loaded processor only with approximately 10-20 percent (Windows performance meter). Uuncompressed video loads processor with less than 5 percent, and the load seem to be stable, while with compressed video it oscillates between 10 and 20 percent on this particular machine. But again, my machine is not dedicated to audio onlly. I have installed various graphic and video applications and this might have slow it down considerably.
The issue with frame jumping at playback start was partly resolved with adjusting frame offset to 4 frames in Nuendo video setup. With this setup I worked on an 40 minutes long audo post project, with many effects, tracks and automation, and as the project started to grow the performance seemed to change, sometimes unpredictably. The frame offset will introduce delay between video and Nuendo and this will show as difference in timecode. Scrubbing and jogging are not affected by offset setting. When exporting finished audio the frame offset should be adjusted to zero again and sync corrected. Thist brings another unncecessary step into postproduction.

My conclusion is that with a powerfull machine dedicated to audio only it is possible to work comfortably with Decklink video. The issues are minimal and can be controlled. I have finished my project using Decklink with ease, keeping the processor load between 50-60 percent. But results will be different on other computer configurations, so you have to know your system well, and still the performance will be a moving target, which may be acceptable or not.
If possible, use uncompressed video, it is more predictable. Try to avoid anamorphic 16:9 video, it will slow down the response of the system.

However, if I would worked with a client present then I would have used Bonsai instead. For serious work (and peace of mind) one machine per task is the norm.