Rita, Thirteen and Air have all been problematic over the years to be fair...
However, considering that Intamin Accelerators are mostly as reliable as a chocolate teapot, and the other two were complete world's firsts (though Thirteen had the issue of poor design, since it was over-speeding)...
Probably other things are seen as unreliable, such as powercuts and how they are dealt with, which is not completely something within the park's control...
Aside from the original 3 rides I listed, the only other ride that breaks down regularly would probably be Spinball... And I don't think Towers have ever had a day where only 3 rides have been operational without a real reason (I.e. powercut)...