Dear reader, I'm not updating these pages anymore. If you have tc or ip related questions, you can post them on the LARTC mailing list.

I found this somewhere on the Internet (original paper). It goes about classes borrowing bandwidth. And isolated is the opposite of borrowing, so maybe it goes about the same problem as I have with the accuracy of CBQ.
2.8 Limitations of Borrowing

    The borrowing mechanism is nice but it has limitations:

    1. a small class cannot borrow the entire bandwidth of its parent.

        a class gets suspended when it is overlimit.
        a smaller class has a longer suspension period (offtime).
        when borrowing is enabled, a child also borrows the offtime of
        the parent.
        but when the parent also gets overlimit, the child has to use
        its own offtime to avoid overloading the system.  (otherwise,
        all the classes use the minimum offtime even under a heavy load.)

        as a result, a small class is not able to make full use of the 
        bandwidth of the parent.

    2. competing TCPs equally share the bandwidth even when their bandwidth
       allocations are not equal. 

        when borrowing is enabled, the bandwidth allocation is
        enforced only when the queues have enough backlog.
        but TCPs can reach the equilibrium without creating backlog in
        the queues.
        in this case, the bandwidth share is made by the TCP mechanism,
        not by CBQ.

        if there are many TCP flows, TCP will not be able to reach
        the equilibrium and the allocation will be done by CBQ.