"In theory" DB are better than barbells because they provide more freedom of movement and require more stabilization.
But in real life they are not always better for several reasons:
1) Insufficient weight (e.g. if the dumbbells do not go high enough when you get strong)
2) Spending more energy bringing the DB to the starting position then lowering them back down when the set is over takes more energy that can actually leave you performing worse during the actual sets.
3) When you fatigue or the weight gets heavy you can become limited by your capacity to stabilize the weight before fully stimulating the prime movers. This is especially true of lifters with less solid technique.
4) DB are a bit more risky, especially when the set is over and you bring the DB down. I've seen a lot of people get injured after their set, not during it.
5) While DB actually provide the opportunity to use a greater range of motion, in real life they seem to be conductive to shortening the range of motion. I see a lot more people doing 1/2 DB bench presses than doing 1/2 bench presses. My theory is that with a barbell you have a visual cue were the barbell is (e.g. on the chest, 1" from the chest, etc.) whereas with DB it is harder to tell when doing the set as the bar is not in your direct field of vision and in the same area as your body.
For comparison purposes, normally the weight you can use on DB movements is 80% / 2 of the barbell equivalent. For example, if you can barbell bench press 315 for 5 reps you can normally DB bench press 125lbs DB x 5 reps. So from experience, one who can do a lot more than that on DBs is likely not using a full range of motion. For example if you can bench press 315 x 5, but can do 150 DB x 5 you are likely shortening your stroke too much.