Dangers of mislabelled immutable functions

I want to index the textual representations of a table's rows.

You can cast a row to text by using the name of the table where you would
usually use the name of a column, like "table_name::text". But this is not
immutable and so can't be used in an expression index.

I can easily make a pl/perl function which does the same thing as the text
cast, and label it as immutable and use it in an index. But are the
dangers of doing this?

I know that configuration changes to the date and time formats could change
the way that the data is shown in ways that would cause the rows inserted
under a different configuration to be missed. That doesn't bother me.

I know that if I drop a table column, the contents of the column will still
be in the index. That doesn't bother me either, as all the indexes I plan
to use are lossy and so will do a recheck anyway.

What other problems am I setting myself up for by doing this?

Cheers,

Jeff