In the UK:
-It is uncodified - easy to amend formally as it only takes an ‘Act of Parliament’, so is flexible.
-Since it is easy to amend, it is also easy to change and update historical clauses that no longer prove helpful or relevant and are seen as anachronistic.
-It establishes a partial separation of powers - a fusion of powers between the executive (PM & Cabinet) and the legislature (Parliament), and a separation of powers between the judiciary and the legislature.
-Establishes a unitary state (structure), though with a growing role for devolved institutions.
-Establishes Parliament (more accurately the 'Crown' in Parliament) as sovereign.
-There is often a lack of clarity and precision in some roles and practices, so convention often becomes very important in the absence of clear and entrenched procedures.
-There is also some piecemeal codification as with the ‘Cabinet Manual’.
In the US:
-It is codified, meaning that its contents are more accessible and better known.
-Being codified also means it is hard to amend as it requires the approval of super-majorities in both chambers of Congress, and also from individual states. Thus, it can be described as rigid.
-Due to its rigidness, it is much harder to remove or revise historical clauses that no longer seem appropriate or desirable in the 21st century.
-It holds sovereignty.
-It establishes a separation of powers - the judiciary, the legislature and the executive are all separate with 'checks and balances' established.
-It establishes a federal state (federalism) - particularly in the Tenth Amendment.
-The Articles of it are very precise in places, for example, the roles and duties of each branch, though some of the terminology can be ‘grey and vague’ - like the Second Amendment: 'A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.'.
In the UK and US, neither a fusion of powers nor a separation of powers is completely embodied.