I think the problem with these products is that they build on the assumptions a file is a file and not that a folder can be a file and a file can be a folder.

The assumptions you need to do to make a folder into a file requires a lot of extra work.

An example of a folder that is actually a file could be a folder named "A Good Movie" with a bunch of .rar files in it.

I've built a streaming plugin for deluge that tries to work around these assumptions but only for a playback scenario and not an indexing scenario. That allows streaming of multi-rar file torrents.

There's also the problem that many rar parsing libraries reads the beginning and end of each part of a split .rar file. That can cause a lot of latency with e.g. 100 files.

Edit: all this applies only to rar files with no compression. With compression you can't do much with regards to seeking and whatnot.