The precedent of most commands is to report failure when the command has no effect. /fillbiome however, always succeeds.

How to reproduce

Run these commands:

/fillbiome ~ ~ ~ ~ ~ ~ badlands
/fillbiome ~ ~ ~ ~ ~ ~ badlands
/fillbiome ~ ~ ~ ~ ~ ~ swamp replace forest

Expected result

Latter two commands should fail: second because the biome was already badlands, and third because there was no forest.

Observed result

Second command succeeds with "1 biome entry/entries set", even though none were changed; third command succeeds with "0 biome entry/entries set"

It would also be expected that result should be the number of entries that were actually changed. Currently, like the feedback message, it includes entries that were already set to the specified biome.