Symbol Timing recovery for modulation producing ISI I am interested in understanding why the common timing recovery algorithms function for modulation schemes which produce ISI.

For example, suppose you are receiving at the output of a matched filter a raised cosine pulse and you are interested in timing recovery. For simplicity lets assume BPSK(though I would be interested in QAM). Besides the ideal sampling times there will be ISI.

Most of the common symbol timing recovery algorithms seem to use symmetry in the received signal, either balancing the measured value at the left and right sides of the ideal sampling instant or use zero crossings.

In the case of a sequence of raised cosine pulses, these features no longer exist as the signals overlap. 

How do timing recovery algorithms work in this case?
In general the ISI is never severe enough for the symbol timing recovery algorithm not to work, but it does degrade its achievable performance on the $P_{be}$ vs. $\dfrac{E_b}{N_0}$ curve.  In other words, for the same error rate, you need better SNR to make up for the ISI.

ISI will degrade the S-curve of the Timing Error detector, causing it to flatten a bit and reduce the TED gain.  Gaussian noise will do the same.