As indicated previously it depends on unique situations. The only real tax issues I have seen is the inheritance tax you could be hit with. If you are both equal then the tax benefits greatly diminish.
However, if your not equal, then there can be some great advantages, everything from being able to fund both you and your SOs ROTH from a single income, being able to take the other spouses SS, being able to combine income and average down your tax bracket.
Right now I'm not married. We have been together 7 years, bought a house w/ joint survivorship. First 5 years I was in a high tax bracket and he was in the 15% tax bracket, no reason to combine and make him pay more for every dollar he made. Now he makes more and I'm retired, but it would be even worse, as right now we are both in the 15% tax bracket, but combining we'd be pushed out and I'd lose my ACA subsidy and have to pay taxes on my dividends.
However, once he stops working (within the next 5 years), then we will likely get married as then we would be living on only my money until we hit 65...as then as a couple, we could convert more of my 401k into a ROTH (since his is almost all ROTH now) and be able to take more money out to live on without pushing out of the 15% tax bracket. It is also likely given his lower lifetime earnings, he will be taking my SS.
Last year I think I figured getting married would cost me $6K in taxes. Thats pretty expensive for a piece of paper and more than I'd have to pay in writing a health directive and such to give us the same legal rights as married people. However, I know once we are both early retired, then marriage is definetly to our benefit.