Generally you want to keep family members out of your marital disagreements because it disrupts the privacy of the marriage. It's as if you're having a disagreement, not just with your husband, but also with his mother and father. It makes it harder to speak up for yourself freely, because you know that anything you say will go back to his family and will impact your future interactions with them.
In addition, things can come up in marital disagreements that are the private information of one spouse. For example, let's say you had an argument with your spouse in which you disclosed abuse that happened to you as a child. If he/she then goes and tells his/her parents, then your traumatic information has been disclosed to someone else without your permission. That would be very problematic. By nature of marriage, ie. the intense and ongoing commingling of lives, the physical intimacy, and the sharing of space where private documents are kept, the things that your spouse discovers about you in a marriage are especially private. For example, your sexual history, your medical information, your financial data, your psychological vulnerabilities. These are things his/her parents are not entitled to know unless you explicitly consent to it being disclosed to them. But if he were in the habit of discussing your marital arguments with his parents, then he might reveal something private in the heat of the moment. Maybe he would be apologetic, but the damage would be done.
There is also the factor that parents are naturally inclined to take their own child's side in a disagreement. This is normal and natural, but, for this very reason, it's not appropriate to bring parents into the arguments of their child's marriage. The end result is likely to be that their partner feels outnumbered in the marriage. It changes the balance of power so that instead of there being an assumption that each spouse has an equal right to speak and to have their interests considered, one spouse has additional "voters" to vote for their interests. Obviously this is not official votes, but the mere anticipation of discomfort at having your private arguments known when you go to family gatherings, exerts its own unfair pressure on the other spouse. In addition, parents (unlike siblings) may also have a sense of authority over their adult child and that child's spouse, that changes the balance of power in a way that may be unfair.
This doesn't mean that a spouse who is struggling can't ever talk to someone outside the marriage. But I think it does mean that those conversations need to stay outside of the social network of the marriage. So talking to your friend who lives in a different part of the country, ok. Talking to a therapist, great. Talking to your mother, no way.
Of course no one should be prevented from disclosing abuse. But that's a special case and also is a separate kind of thing than just discussing your generic marital arguments with your mother, because the expectation, in cases of disclosing abuse, would be immediate action to rescue the person from that situation.