I think the Borg, as a concept, somewhat falls apart when we considering that natural, biological systems are actually often perfect models for the efficiency that the Borg claim to strive for. And, to clarify, I’m not saying the concept falls apart from a doylist perspective - I think that the fact that Borg technology evolves independent of any particular intent and is highly automated to take the most efficient route to its endpoint kind of reveals the folly of the Borg, which would be super interesting to explore. They’re just recreating systems which already exist in nature, from a certain point of view.

In Trek, at least, they seem to be less efficient, but more adaptable. A computer, even in Trek is faster, more responsive, but unless it is independently conscious, lacks the flexibility to work outside of its programming, occasionally in self-destructive ways.

This is particularly apparent when it comes to dealing with inherently-illogical things like emotions, which might cause the them to shut down permanently (Rayna and Lal), or go slightly mad (Lore, and M-5).

The Borg seem to be trying to get the best of both worlds, by trying to keep and cybernetically augment existing biological systems, which would ideally keep their adaptability, whilst adding the durability and rapid processing of cybernetics. It might work, but they're held back by both limitations of their technology, and their nature as Borg.

Because the Collective does not recognise individuality, the only way that they can improve is via assimilation, and there aren't very many species that would allow themselves to be assimilated by the Borg, whilst also being their technological superiors. They don't really foster the kind of environment that would allow them to improve their own technology, absent any external pressure/input to do so.

Considering the miracle of dermal regenerators and similar technology, I actually think Assimilation is highly reversible. Just still really traumatic.

Although from what we see in Picard, that seems to be a Federation technology. Other species don't have quite that advanced medicine, and the changes are rarely so surface-level. If First Contact is to be believed, there are deeper, underlying changes that may not be reversible, and in Voyager, ex-Borg drones that have been drones for long enough can't be completely unassimilated. The cybernetic parts in their bodies are beyond even the Federation's ability to safely remove, and are dependent on the other Borg hardware to function properly.

It might be possible to use a transporter to extract all of it at once, but the resulting shock might just kill them instantly, if it didn't leave lethal wounds.