To the extent people are worried about legalities, in many cases there will be meaningful legal consequences to putting money or investments in a joint account. Whether a state is a “community property” state or a separate property state, in most states in the case of a divorce either spouse’s earnings from employment during the marriage (and savings from those earnings) will be treated as marital property subject to division in the divorce proceedings – either 50-50, or “equitable” – but other property (and earnings on it) may be treated as separately owned and not subject to division at all, but often only if it has been kept separate throughout the marriage, and not been used on a regular basis to maintain the household on a regular basis. That will usually include property and savings owned before the marriage, and inheritances and gifts received by a spouse separately. It’s that stuff you really have to be intentional about.
Of course, the point of most financial arrangements we are discussion is to avoid divorce, not to plan for it! In that sense, for many couples maintaining separate finances can make a lot of sense, even if the separation may not be respected fully in the event of a divorce. In that case, however, it may be wise for people to have two separate accounts for each spouse – one for truly separate, non-marital property, and the other for the portion of your paycheck over which you have sole dominion.
If any outsider looked at my family’s bank accounts, we would look like a “your, mine, and ours” family. In fact, however, we commingle everything, including money inherited from my parents. Historically, I have earned 70%-80% of our income, although there were a few years (bad for me, good for her) when it was close to 50-50. Our separate accounts are really an artifact of my wife managing the finances, and my not necessarily going to the bank with her. I have one moribund separate account that we should terminate; she has a separate account through which most of our bills get paid. That’s not how I would tell a client to do things!
My parents paid for my college and law school, so I was debt-free. My wife’s parents paid for her college, and their divorce agreement specified that her father would pay for her law school. But neither parent bothered to tell her that, and we didn’t find out about it until after they both died 35 years later. (Her mother assumed her father was paying; her father was being a jerk. She didn’t discuss her finances with either of them.) She paid for her first two years of law school out of savings, loans, and summer job earnings; we got married before the third year and we paid for that, and repaid the loans, mostly out of my earnings. Thanks to the high inflation at the time, and my earning capacity, it wasn’t that much of a burden.