How does devolution in the UK differ from federalism in the USA?

There are two core differences between devolution and federalism:

1. Devolution is done on an asymmetrical basis in the UK. This means that Scotland have some powers that Wales does not and Scotland and Wales have powers that Northern Ireland does not have. In the USA, all 50 states have the same powers.
2. In the UK, the Westminster government can take back devolved powers at any time. It has done this on a number of occasions in Northern Ireland. In the USA, the 10^th Amendment of the Constitution guarantees powers to the states that cannot be taken away. Whilst there is often disagreement between the states and the federal government about who has power in certain areas, there are clear policy areas such as making criminal law that are left to the states.