There is one downside, I know of, to joint accounts. If you do this you must both name the same attorneys in any Powers of Attorney you make, because your attorneys can only act for the individual and with joint accounts, any action by an attorney affects the assets of the joint account holder, for whom they are not attorney.
I had to deal with some elderly relations who made their PoAs separately with different attorneys appointed and then made all their bank accounts joint.
Thankfully, when they were both diagnosed with dementia , we discovered that one attorney was common to both, but, as a result, the full burden of financial management fell on her as she was the only one who could act for both in relation to the bank accounts and income that paid for their needs and care.
I was one of the other attorneys, I could make all kinds of decisions about welfare etc, but as soon as money was involved it had to be sent to the joint attorney. It did cause some problems.