Builders are experienced (if not experts) in their field and you will be thankful for their knowledge and experience. You cannot assume that you can learn all there is to know in the space of time before you start your project: it is inevitable that you will make mistakes, that your build will progress more slowly, that the scheduling of works and co-ordination of trades will be relatively poor by comparison and that this will all cost you time and money.
How much depends on how well you can learn on the job. Don’t be pennywise and pound foolish – be brutally honest and realistic about your abilities before you take the plunge to go direct labour. Time is usually the big factor. If you have a full-time job which prevents you from being on site when work is being done, then it is inevitable that some things will be overlooked.