It is the best part of real estate because it lets the agents with experience shine who know how to place value on a home using the sales approach. Knowing how to search for, spot and choose the right comps applying the correct adjustments where needed. 

I'm with Scott Godzyk on this one.  

No hard part here

Often, there are just no comps available as many target communities are stabilzed and homeowners rarely sell.

The hardest part is that information in MLS listings may be outdated, incomplete or just plain wrong. Market analysis is truly one of the most difficult tasks we perform. A new agent recently asked me whether she should 'waste time' doing BPOs since they are involved and pay little. I told her that is great paid training and will sharpen those skills big time!

jacquelene rivers As Scott Godzyk stated, using the correct adjustments is the difficult part. The adjustments can make or break a CMA.

Knowing the area the subject and comparable properties.

You will not know the condition of the sold homes.  Sometimes there may not be comparable sales.

It is like so many tasks, if you know what you are doing, it is easy.

If you don't, the guessing does create uncertainty.

Because you limited the option to 'sales comparison'  a lack of comparable sales would seem the greatest issue.

Making certain that you have selected the best possible comparable properties.

knowing what the real condition is

Picking the correct and most accurate comps to use.