The common ash can be a good bonsai choice as long as you consider a number of factors. As with any tree you have positive points but drawbacks also.
1. Leaves are made up of lots of leavelets which can make it look bare in the winter if you do not have enough branches
2. Because of the large leaves it needs to be made into quite a large tree
3. Leaves appear very late in the year, they are one of the last trees to come into leaf
Here is a picture of a fully grown tree at the golf course I work which shows how a bonsai could look. It’s a little hard to see the outline of it because everything is so green there at the moment but you can see it is a grand tree indeed. Ash trees are as hardy as old boots. You can do anything to them and they will just keep on growing. I am growing a yamadori ash at the moment which I am giving updates of on this site.