Hi Chris - often times text can have badly formed curves, which don't affect their original purpose of 2D printing but can be bad for 3D geometry purposes such as filleting.

In your case here there are some small sized segments which then converted into very thin slivery faces in the extrusion and those will complicate filleting.

Here I've circled and zoomed in some to show you the problem areas:






The filleting will fail if it would cause a face to be totally consumed away to nothing, that will be easy to have happen with such small slivery faces in the shape.

Then the other thing that is difficult for filleting is dealing with very pronounced angles between pieces which is common with text.

These issues can tend to make filleting of text to be difficult.

- Michael