Don't the Voth have personal phasing cloaks, and can transport entire starships at once?
The Borg don't seem to have those things, so it's unlikely they've been successful in their assimilation attempts.
Yes, from what we saw of the Voth, they were technologically superior to the Borg. I have suspected there may be reasons Borg space in Seasons 3-4 had borders where they were. *Something* is keeping the Borg from venturing much further outward from "Scorpion" space. The only people with fear of the Borg were that one Sakari mining colony that went underground. The Voth have so much the Borg would love to get their hands on, the Nyrians had advanced transporters (Borg transporters are only close range, Nyrians could teleport, albeit one at a time, over a distance of 10 ly). Nyrians also had some advanced tech with accessing computer databases across light years and decrypting it. And they are almost on the doorstep of the Borg there too. I suspect that something was the Voth broadly roaming across that band of space between the Nekrit Expanse and Borg space. As noted, they have been space faring for millions upon millions of years. The Borg Collective was very small 1000 years ago. They did seem to be growing rapidly of late though (probably critical mass with numbers and new technology acquired from others. That probably is what led to Arturis' race finally falling). We still haven't seen what the Borg might use those tactical cubes for, seen in "Unimatrix Zero" (I don't think it's merely precautions after Species 8472. Seven was familiar with it and the sense I got from it was that it wasn't some new invention). Each quadrant is huge. There are obviously races, some empires, more powerful than the Borg who, while they may lack the numbers, may have a technological edge over the Borg. It seems a bit naive to think the Borg may be the most advanced race in the galaxy. Most numerous... possibly.
On the other side, the decisive Borg border seems to be 50~100 ly before the drop-off point in "The Gift". There's a transwarp conduit through part of the region (B'omar/"The Raven") but no other Season 4 race shows fear of Borg and later on, Seven if unfamiliar with some species (like the Dream Aliens, who most certainly would be assimilated because the Borg would most definately be interested in their Unconscious/Subconscious Collective). I suspect the Krenim form a barrier on that side. The Borg have advanced sensors. They likely could pick up all the temporal anomalies around Krenim space. Annorax was on a quest to get the empire back to 100%. If the Borg ever encroached on the Krenim Imperium, Annorax would have blasted the Borg world of origin out of the space time continuum. Their weapons and temporal science would obviously be of interest to the Borg but if the Borg detected races blinking out of existence, they might be cautious to venture there.
LOL at people claiming they may be unworthy of assimilation. Some are, like the Kazon, but if races offer something, technological or physical (big dumb brutes were sometimes mentioned as making good tactical drones), they will be assimilated. Hard to tell what makes the Borg to go all in (like on the Brunali) vs. just casual assimilation (like the Norcadians). We've seen many races in Voyager with *something* worth assimilating. Even the annoying munchkins from "Virtuoso", the Qomar had something of value (complex algorithms, some of the most advanced mathematical skills seen in Star Trek). Interestingly, many of them apparently haven't been decimated by the Borg. Overall though, the Borg don't seem to explore very far from their transwarp conduits. That's how Brunali gets decimated but Norcadia and other worlds they casually assimilate some vessels from but don't go all out.
BTW, all species get numbered. Even the Kazon had a number. Species number seems to not be species of value necessarily; it's the Borg catalog of species they encountered. Even some primitive societies had members assimilated (the ones with the Omega stories). The Borg probably assimilate the unlucky aliens who are the first the Borg have ever encountered of their race. Species 8472 was given a number and they obviously could not be assimilated.
The thing about Borg space is I have wondered how many holdouts like Arturis' race were there? Races that were more advanced but besieged, fighting a gradually losing battle? And how many worlds of pre-warp civilizations or primitive societies are there in Borg space, left untouched because they don't have any real value or minerals to plunder (which was probably why those 2 Borg ships went to the Nekrit Expanse. Most planets encountered there were mining worlds and the Fair Trade station made mention of lots of mining colonies along the edge of the expanse. Remember, the Borg would also 'assimilate' non-lifeforms, non-technology, like worlds with desirable mineral resources. They were also after a very rare mineral to produce Omega particles).
. One of the things I love doing with Voyager, like the Equinox distance paradox and their very different journey)
