Mestiko's name-checked in A Less Perfect Union as being one of the worlds the Klingons have picked off, along with Organia...
Just to clarify, that's in an alternate timeline, in case anyone was confused.
A bit of perspective: in this part of the galaxy, a sphere 100 light-years in diameter might contain something like 15,000 stars. Even if only one in ten were inhabited, that's still 1,500 populated systems. It seems unlikely that all those could be wiped out in the space of a few days.
It doesn't to me -- not when you're talking about an invasion fleet of over 7,000 cubes that can apparently each exterminate a planet in less than an hour, anyway.
I think we're shown several cubes at least in each system. And many of those systems might have multiple inhabited worlds or moons to deal with -- Sol System has Earth, Luna, Mars, and colonies at Jupiter and Saturn at the very least.
Yes, thanks for the reminder. Sol got 12, I think, Vulcan 10, Qo'noS about 10 (in the first wave), Andor 8. I guess the Borg, while destroying everything in reach, wanted to ensure they dealt with the major planets, which would have the heaviest defenses, and invested more ships for those targets.