For one thing a lot of tools like Excel do unwanted data transformations, such as importing from a CSV they try to guess whether text is meant to be a string or a number and sometimes guess wrong. You can spend a lot of time disabling this behavior or preventing it or fixing it up afterwards, but frequently bad data just plain corrupts the analysis or target system.
You can't really trust the time handling functions on any platform which is some of the reason why languages like Python, and Java, might have two or three libraries for working with dates and times in the standard library because people realized the old one was unfixable. Plenty of Javascript date handling libraries have told people "look, this is obsolete, it's time to move on" not just because that's the Javascript way, but because the libraries really were error prone.
In a real programming language it's straightforward to fix those problems, in a fake programming language it is difficult or impossible.
If you've got a strange index structure in the source or destination data, for instance, many tools fall down. For instance if you want to convert nested set trees
https://blog.uniauth.com/nested-set-model
to something more normal like an adjacency list (or vice versa) a lot of simple tools are going to fall down.