NEC ECC: Are there any disadvantages from employer perspective of leaving tenderers to define activities?

Currently tendering for the construction of a parking area for some new commercial buildings which is to be awarded using option Employer has left us (tenderers) free to define activities, just wondering what the potential benefits may be from employer perspective in using this method as opposed to pre-defining own activities?
What are the drawbacks from pre-defined activities?

The dangers is you end up with an activity schedule that does not match how the Contractor would like to or actually is going to do the work. So far from getting transparency over costs, you end up with confusion.