COALESCE is fair better than isnull and I make my DBAs use COALESCE.

Example

DECLARE @a TinyInt

SELECT ISNULL(@A,256)

This will lead into Arithmetic overflow error for data type tinyint, value = 256.

But not with

DECLARE @a TinyInt

SELECT COALESCE(@A,256)

but if you want your query to fail then obviously use isnull