I haven't used a mandrel in over 10 years so I can't answer that. But I will offer you a link to a 10 year old thread from this forum that is still very helpful:
I am on a fact finding mission. Can you help? What do you wish you had known earlier in pen turning? or in different words: Knowing what you know now - IF you could go back and start over in pen turning - what would you skip, change or do differently? answers for instance: - I wish...
www.penturners.org
At some points in there, TBC - Turning Between Centers (Not using a mandrel) is mentioned and that method is well used by many here. Mandrels work well for quiet a few, but there are problems for those unfamiliar with tracking down problems such as not concentric, wobble, oval. But if good at problem solving, and one likes to do both parts of a two part pen simultaneously, mandrels work well.