It seems like there is a lapse in logic if you believe having a high MMR should mean you win every match. I do not think the majority of the people who complain about this situation understand what MMR is supposed to do and what it means having a high and low MMR.
Any game that uses a matching system will eventually match you to a match you will not win because as you increase your MMR you are telling the game you are better than X% of the people who are also playing. Just like MMRs work for every game, it is a measure of your skill in relation to other players.
In this case the "other player" is the AI and it seems logical that it should become increasingly challenging as you increase your MMR.
I mean, you can't get a score of 10 at any MMR as far as I know. And even at ~30% MMR you will get a 9.2-9.3 with a perfect score(even less than perfect). You should logically infer that the game was balanced for ~30% or whatever the lowest MMR is that you can also achieve a 9.2-9.3 with a near perfect performance.
Then you can draw the conclusion that having ~20% OVER the difficulty the game was designed with then you are attempting an extra challenge for largely your own personal gratification. There are very very minuscule rewards for getting your MMR high and the reason for this is super duper obvious. It is stupid easy to get a high MMR.
MMR is for the challenge, if that is your thing. It does not seem to be anybodies thing, however. Well, you're in luck as it is stupid easy to lower your MMR.
The idea that the game isn't being balanced correctly comes from this misunderstanding about the nature of the MMR system. Many people fail to realize that just because the game lacks a sophisticated PVP ranking system doesn't mean the rules have changed at all regarding the way matching is set up. You will eventually, like every other game, get a match you can't win. Just because it is against the AI shouldn't matter either way.
Unfortunately, no one will see it this way. They will relentlessly slam their head against the MMR wall wondering how they, who are so very good at the game, could possibly lose. Then come here and strongly imply the game is too difficult without admitting it is their own lack of skill preventing them from claiming the victory they feel they are owed.
The problem is the game is being balanced towards pvp as well. So guns eventually don't deal enough damage to take care of the enemy hordes and your hp doesn't scale up either while enemies damage/health just keeps going higher. Eventually at around 60%+ mmr it becomes too much for raiders to handle. They need to remove the mmr system and just add difficulty modes that are properly balanced for the current raiders strenghts.
Right now the enemies are bullet sponges with the exception of the few guns out there. You're better off Meleeing, even then the enemy has a higher chance of countering. The raiders are squishy including fith council.