Is awful. Wargaming basically said “well, we fucked up with the BZ-176, better remove those derp guns and make sure the line is useless otherwise”
Lets start just overall: these tanks are SLOW. The best power/weight ratio in the whole line is 13.58 at the tier 7, and it only goes down from there. This is only marginally better than superheavies.
At best, your tier 10 has six 10 second boosters that will bring it up to 45-50kph, but only in a mostly straight line. There are many tanks, even other heavy tanks, that can hit this speed under normal engine power without a limited, gimmicky mechanic to achieve it.
On to specifics:
The Tier 7 (BZ-58):
Overall weaknesses notwithstanding, this is the most balanced of the tanks in the line compared to its tier. However, tier 7 heavies are pretty notoriously bad at this point, especially with the exceptional power creep at tier 8 they face so regularly. This slug fairs no better.
The Tier 8 (BZ-166):
This might be the worst in the line. It has pen values that fall in the average range for a heavy of it’s tier, but dispersion values more along the lines of preferential matchmaking tanks (IS-6) or dedicated brawlers (Defender), but coupled even moreso with the atrocious DPM of high pen, high accuracy heavies like the Lowe and T34. It takes too long to reload to brawl, its accuracy is too poor to snipe, and its pen is too poor to ensure that it at least pens every shot. Couple this with a marginally above average alpha, and you have an extremely poor gun platform.
Slap on top of that an armor profile that is only designed for engagements dead ahead, but still has an easy to pen weakspot in the center of the upper hull and two cupola weakspots on the turret, one of which is fairly large.
The Tier 9 (BZ-68):
This tank is more in line with it’s tier, however with a few glaring weaknesses. Compared to similar tanks, its dispersion is much worse, with the exception of the Obj 705 (which no one will argue is particularly strong). However, it suffers from the same “front towards enemy” armor layout of the tier 8, making angling extremely risky. It does loose the hull weakspot and get slightly reduced size cupolas, but they are still easy to pen.
And again, this thing is a total slug at its tier. It only really excels when fully depressed in the proper position, but unfortunately due to its mobility, maneuvering into and out of that position (e.g. peeking a hill) will leave it vulnerable. And even then, even slightly angling to its sides opens up easy to pen weakspots.
The Tier 10 (BZ-75):
Another stinker. With the exception of standard pen, the gun is worse in nearly every way from every other 650/750 alpha heavy at tier 10, except the 705A. But again, the “front towards enemy” armor layout that still persists means it cannot brawl as well as the 705A, and definitely not as well as every other tank that hits harder than it does. On top of that (literally), the turret has 3 massive, easy to smack weakspots, still fully visible at max depression.
And finally, it maintains the same awful top speed of 30kph when non-boosted with a terrible p/w ratio, making it a total slug on the battlefield. This tank continues the trend of trading being good at anything for temporary, finite rocket boosts.
The whole line sucks.