Disadvantages
Each equinox and solstice varies over a range of at least seven days. This is because each equinox and solstice may occur any day of the week and hence on at least seven different ISO week dates. For example, there are spring equinoxes on 2004-W12-7 and 2010-W11-7.
It does not replace the Gregorian calendar, which it uses to define the new year day (Week 1 Day 1). However, it could be defined without reference to Gregorian. One needs at most a defined start and a table of year-lengths over the 400-year cycle.
Not all parts of the world have a work week that begins with Monday. For example, in some Muslim countries, the work week may begin on Saturday, while in Israel it may begin on Sunday. In the US the work week is often defined to start on Monday, although the week itself is usually considered to start on Sunday.
Read more about this topic: ISO Week Date