Symbol Timing recovery for modulation producing ISI
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.