I have been commissioner for 20 years. I do have a veto clause in the rules, but I told every owner that I will only utilize it if collusion is suspected. If a trade looks completely lopsided, I follow the following steps:
1- I call each owner and have them explain their logic. Most times it ends here as one or both owners see things from an angle I didn't think of.
2- If I'm not satisfied, I'll bring in the two other most senior owners and we'll discuss.
3- I can involve a league vote if I feel it's needed. I have only used this one time when collusion was suspected. It was followed by a vote to kick an owner out.
The only other time I will step in the middle of a trade is if we have a new owner that is clearly being taken advantage of. We have a fairly complicated league and it usually take a year for the average owner to "get" the value of IDP's. This has happened a few times, but was resolved with a couple of phone calls and a modified trade offer.
I believe that each owner has the right to run their team as they see fit. And that means taking the consequences of a bad trade. That's how you learn. Also, I think trust plays a huge role. We don't intentionally screw each other over because we have played many years together. What comes around-goes around. If you intentionally tried to screw someone in a trade last year, you will find it 10 times more difficult to trade this year just because of your reputation.
The underlying problem is player value. Every owner values each player just a little differently. So every owner is going to value any trade differently. This, along with motives to "not let another team get stronger" is a perfect reason to not let voting on trades occur.
Since I took this stance many years ago, there have been no trade complaints other than the one that involved collusion. You would be amazed how owners can act like adults when you allow them to. Sorry for the ramble. Too many late night beers.