Actually Tashi, I believe it's more difficult for most users to trouble-shoot the complexities of different products when they are mixed together. This varies of course based on how well the particular products remain focused in one area of protection. For example, Spybot - Search and Destroy does an excellent job of staying within the realm of antispyware protection.
In truth, suites should actually be the best type of protection for those with less technical knowledge, since the individual components can be tuned together and tend to avoid the potential for conflict and false positives with other components. With individual products it's up to the person installing them to determine whether the products overlap and thus might create conflicts and/or confusion between each other.
In practice, however, suites have a negative that they have generally created for themselves. The problem is 'feature creep' or the tendancy to add features, often with little or no real security value, to increase the sales potential of the suite. By doing this the potential for more sales does tend to increase, but so does the complexity and the general 'bloat', reducing the overall effectiveness of actual security protection over time. This might be what Tashi is specifically referring to, since over time almost every suite has had this occur, resulting in the need to 'start over' with a trimmed down version removing much of the bloat.
The other issue with suites is that their very design can increase the available attack surfaces that malware can use to cripple the protection. Again this isn't necessarily inherent, but rather a side effect of trends within the design itself. For example, the tendancy to make a single control screen for configuration of all components of the suite also creates a potential attack point that might be used to cripple the entire suite. There are other less visible shared points of potential attack and failure, but this doesn't inherently mean they must be vulnerable to attack, though they can be if this possibility is ignored by the designers.
The key issue for most users is to find protection they can understand, whether it's a suite or a combination of separate security applications. The most common failure today isn't due to the products themselves once they're properly installed, but rather the user making a mistake due to either confusion or simply ignoring what the protection is telling them. This is the difficulty for all products, since no matter how good the protection, if it is ignored or misunderstood the result can still be an infection.
Bitman