If you provided a lovely full environment with lots of activities to choose from at an appropriate level, I'd say unschooling would be like Reception, except maybe that in Reception the children have blocks, painting, and prewriting skills set up and they are gently encouraged not to do blocks all the time, whereas in unschooling they would be allowed to do blocks all the time if they wanted to.
My experience from a family member "unschooling" is that the paints are in a cupboard, the blocks are out, the TV is available or on all the time, there's nothing much to do prewriting skills with let alone ability-appropriate worksheets and easy grip pens, and the children therefore do blocks or watch TV.
Children are very good at doing the same thing again and again and if that's what's available they will follow the path of least resistance.
Older children who ask a question about something may be able to retain interest while the parent sets up a learning opportunity but if a 5 year old asks about Egyptians but the parent has to wait till the afternoon to go to the library or the next month to organise a trip to a museum, they will probably be back to blocks.