Taiob Ali points out something added to SQL Server 2017 (and later 2016 and 2014):
Microsoft SQL Server Management Studio (SSMS) version 17.5 added new showplan attributes UdfCpuTime and UdfElapsedTime to QueryTimeStats. These two attributes will measure the time and CPU spent on user-defined functions within a query execution hence helping to discover the impact of UDF execution within full query execution. This feature was first added in SQL Server 2017 CU3 and was backported to SQL Server 2016 SP2. Finding the execution time and CPU for UDF was always a challenge for Data professionals because the number of times a function will execute will vary.
This was a blind spot for a very long time.