So many matches since S4 are so imbalanced it is crazy. So much domination almost every match. Is the MMR broken, is it something else. Is it me? I'm not the carry, and I used to win...now it's like 90% losses since s4. I played all weekend and it was just frustration the whole time.

It's the lobby balancing system. From my interpretation of what I have looked at online, as I feel that I am in a similar boat, that the system will prioritize putting us in lobbies, on the losing side, due to our stats indicating that we won't do "worse" on a losing team.
The system will not want to put a player such as yourself who may go 25-10, with consistent flag captures, onto the winning team, or a team that has players of similar skill. Instead, you will be put on the "losing" team as a method to potentially even out the playing field.
From an anecdotal perspective, quitting as soon as you begin to notice a loss (for me it is if we lose caps too quickly in breakthrough) appears to reduce the chances of being put onto the losing team. This for me is what I would call a brute-force method, however, since it's not addressing the issue itself, but instead attempting to circumvent it.
There also is one instance where I quit a specific match, and was put into the same lobby, except then on the opposite team.
My W/L rate is 48%, however I "feel" like I am losing more because I sometimes opt to not leave a match, and therefore wasting time. Quitting matches does not appear to actually affect your stats regarding winning or losing matches, so don't worry about getting a penalty for now.

Just lost 6 rounds in a row. We didn't just lose, we were absolutely dominated...

They blocked the good Rush maps!
Game is broken.

yes

I'm a person who almost never quits a game, no matter how much I see my team losing. Checking my team's scoreboard, there are always 2–5 people "..." and quit. Sometimes I load into a match that's already halfway lost, or play for like 2-4 minutes for the game to end in a defeat, because got backfilled into a spot left by someone who quit. But I am not a PvP player(I'm PVE player but no PVE BF style games), winning or losing has no effect on my mood any game while playing. My win rate is at 52%. Probably low total match count, so my winrate hasn't normalized to 49–51% yet.
I will only quit at the start of the game when I load in and player ratios like 4vs12, 1vs8, or 8v20, 10vs24 which result from bots filling in during player search and the early game not balancing properly. This is commonly seen during off-peak hours in my time zone, such as when I play from 4 AM to 7 AM. I've seen how broken their matchmaking is for a long time. I must say this game barely cares because it does not have a serious rank system.
This game allows you leave matches to protect your win rate, so you can seeing 1k+ games with a win rate 90%+.

The desync from console to pc is the biggest killer of my enjoyment. If i got every kill i should have if my last bullet actually registered on there slow servers my kd would be much better. Too many time gun fights i should have one favour the crappy desync.
Its getting beyond a joke now. Also for some reason at rank 317 i keep getting put in beside pre rank 100 players. Like loads on newbies at ranks under 50. Then next match im in with dudes at 500+ does not make sense.

Short answer the game is broken. Have you seen all the bugs? Longer answer is I don't know what is going on but the hardest weekly for me to get was to win 3 games in a row. I was logging on/off. Try reboots and this ended up spilling into next week so something like out of 50 games I won 3? I have noticed a lot more 100 kills to around 5 deaths players on Breakthrough which has me thinking something is suspect. However at this point I really just don't care. The game is not fun so I usually quit after 3 matches, occasionally I do get a match and the bullets work and I am happy with how it flows. However that is very rare. I mostly play solo as well so that may play into things.

Having insane desync issues for the past three days. I've never had matches this bad where I'm in the bottom five players like this. I'm constantly shot around corners and instant dead no matters where I go. Something is not right.
I want to win, of course, but having fun is better. It's not fun when there's so many technical network issues going on with the game. Nothing fun about being around corners for some time and the game still registering that I'm being hit.

Just lost 6 rounds in a row. We didn't just lose, we were absolutely dominated...
It's been that way since the Battlefield 2 days

Yes, I fully understand, the same thing happens to me but there are other things that need to be improved, such as the crossplayer, improved user reports on the SEC network, among others.

Short answer the game is broken. Have you seen all the bugs? Longer answer is I don't know what is going on but the hardest weekly for me to get was to win 3 games in a row. I was logging on/off. Try reboots and this ended up spilling into next week so something like out of 50 games I won 3? I have noticed a lot more 100 kills to around 5 deaths players on Breakthrough which has me thinking something is suspect. However at this point I really just don't care. The game is not fun so I usually quit after 3 matches, occasionally I do get a match and the bullets work and I am happy with how it flows. However that is very rare. I mostly play solo as well so that may play into things.
I've seen 2 bugs:
Win or Complete(unsure) challenges bug out and won't trigger until a game restart, but it completed after restart leaving me unsure if I even got the reward.
Bonus Challenge already completed before play (so I miss out 10 point), but then i played 20–40 games later for farm battle pass after 100% Challenge complete, and suddenly got 10 points, as if the Bonus Challenge was completed.
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If you ask me what feels the most broken, it has to be how they designed Breakthrough from a large-scale mode into a small-scale.

Not really sure how they match players tbh.
Most matches I have a negative KD, however I'm usually in the top 10 on the scoreboard due to having high captures. So I'll look something like 7/10/30/8 from being spec ops recon, keeping a tugs/jammer in the frontline near objs at all times and trying to capture/keep objs at all times.
My W/L ratio is like 40% Wins and 60% Losses. I always end up on TDM teams or teams with players that have no idea what is going on. They're basically just running around like chickens with their heads off as if it's their first match at level 120.

The balance has become so bad its not even fun playing the smaller infantry maps anymore. Cairo is the worst now. Most of the games end half time with the winning team having all the capture points and other team stuck in spawn getting smashed to bits. The larger maps are a bit better as the "weaker" team has time to flank and take capture points without getting bulldozed.