It’s not great for biodiversity - and in the long term can be quite suppressive, especially if it establishes big thickets.
The understory becomes very barren, with an invertebrate fauna that’s more or less entirely dominated by spiders and very little leaf litter or soil fauna. Some birds do nest in it, but many do not - it essentially acts as an ecological homogeniser, and you can lose things like willow warblers, white throats, black caps and the parids if it becomes too dense and extensive.
It’s also terrible for suppressing other plants. On heathland, it’s notorious for leading to declines in many rarer, more interesting plants. I’m involved with a long term project on fragments of dry heath on SSSIs in Fife, and they’ve been very badly affected by gorse. Several have more or less completely lost all the original rich flora and are now just gorse monocultures.
Like most things, a bit is fine, too much is not.