What would make a parallel approach difficult?

What would make a parallel approach difficult? Paraphrase: Why could a parallel approach be difficult?
Parallel approach operations require every pilot good situational awareness. A thorough review of the Approach Chart should be conducted with great attention on all approach information.
The close proximity of aircraft conducting approaches to parallel runways demands strict and timely compliance with ATC assigned speeds, altitudes and headings. Precise final approach course tracking is critical to maintaining required separation. Pilots should notify ATC immediately in the event of loss or degradation of aircraft or navigation systems.
Strict radio discipline is really obligatory during simultaneous parallel approach operations. Long enough or unnecessary radio transmissions must be avoided. Full and proper call signs should be used to prevent the mistaken clearances intended for another airplane or confusion due to similar sounding call signs. 

Traffic Collision Avoidance Systems (TCAS) provide an additional element of safety to parallel approach operations. Pilots should follow recommended TCAS operating procedures in the event of a Resolution Advisory (RA).

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