Most of the following was posted here but I've put some more thought into the topic and have come up with an explanation why tier 8 MM sucks right now. The following is more or less guesswork and I'd like your opinion on it. TL;DR below.

• Tier 10 has awesome MM now (3/5/7 means few tier 10s on the enemy team, lots of tier 8s to shit on)
• Tier 8 (and to a lesser degree tier 9) has shit MM (seeing mostly tier 10 games, but the 3/5/7 rule means that its not as bad as it used to be, facing 12 tier 10s in your stock IS-3 is now impossible)
• Tier 7 has awesome MM

I havent played too many tier 10s since the patch, but I played alot of tier 8s and 7s. Namely i played through the challenger and its been a breeze since i was top tier nearly all the time. now im on the charioteer and face 10s constantly (thank god 3/5/7 rule and 268 standart pen)

my explanation:

tank appearance per tier, last 30 days, from vbaddict.net:

• 5: 4,823,924
• 6: 6,961,367
• 7: 6,029,954
• 8: 13,308,462
• 9: 7,429,616
• 10: 4,751,424

notes:

• the numbers are not totally correct because as of today, may 13th, the 30-day period used includes the last week before patch 9.18 was released. tier 9s will have appeared more frequent since the patch because many people trying the new tier 9 lights and grinding them for the tier 10s.
• tiers 5 and 6 are probably under-represented here because these numbers are based on games uploaded to vbaddict. new players at lower tiers dont do this and experienced players that do upload their games to vbaddict often play higher tiers more frequently.

I think the old MM created games according to the number and tier of tanks in the queue. Since Tier 8s were played more often than 9s and 10s combined, they had a >50% chance to be top tier.

The new MM seems to start at the top, i.e. tier 10, create a game and try to fill it following the 3/5/7 rule with 10s/9s/8s. The numbers above suggest that tanks of tiers 8 to 10 are joining the battle queue roughly in a 3/5/7 ratio with a bit too many 8s. this means that tier 10s will usually be top tier in a 3/5/7 match, with almost never a 5/10 or single-tier matchup (except during low-pop times at night). Having satisfied the tier 10s leaves very few tier 9s but still a considerable number of tier 8s in the queue.

The MM not only tries to follow the 3/5/7 rule but also to balance the individual vehicles according to their MM weight (strong tanks have heigh MM weight, weak tanks have low) and their class against each other. The small pool of tier 9s to pick from means longer queue times which the MM counters by throwing the 3/5/7 rule out the window and resorting to 5/10. This requires more tanks from a small pool, but gives more flexibility to balance the limited options against each other. There are now only a few tier 8s left in the queue.

Tier 8 has the same problem like tier 9, namely a small pool of vehicles in the queue to pick balanced teams from. When the MM falls back to 5/10 it encounters another problem: there's actually rarely enough tier 7s in the queue to pick balanced teams from. Why?

Another rule of MM is to keep queue times as low as possible. This means most tier 7s are instantly thrown into tier 7 games and when the MM tries to create a tier 8 game there's actually not enough tier 7s to pick 20 balanced tanks from. It will eventually fall back to single-tier tier 8 games.

TL;DR: MM is bent on creating tier 10 games following the 3/5/7 rule meaning most tier 8s are put into tier 10 games right away. The remaining 8s cant get into tier 8 3/5/7 games because while there are many tier 7s in the queue they disappear into tier 7 games that only require 6 of them to create a 3/5/7 instead of 14 or 20 for a tier 8 3/5/7 or 5/10 game respectively. This means if you dont get thrown into a tier 10 game right away your tier 8 tank will most likely see a single-tiered tier 8 game.

the more people realize how good tier 10 and 7 MM is, the more tanks of those tiers will join the queue. At the same time we will see less tier 8 tanks because, frankly, their MM sucks right now. This means over time the system will stabilize and tier 10 will see more 5/10 and single-tier games while tier 7 might get popular enough to always have a large pool of tanks in the queue to provide better MM for tier 8.