It has many of the advantages of varnish, with few of the drawbacks. It flows better than varnish, so brush marks are more likely to disappear. Correctly applied, polyurethane is more durable than varnish. Since they don't like the way the top looks, and they've been told it was polyurethane, they don't like polyurethane. If you take three identical pieces of furniture, finish one in lacquer, one in varnish, and one in polyurethane, no one is going to be able to tell you which is which without testing the finish with solvents.
Polyurethane lends itself to good results with a minimum of investment.
It's its own sealer, and you don't need a fancy brush to get good results. Foam brushes give great results. That's a lot cheaper than a fifteen or twenty dollar brush, which you have to clean after every use. The professional will invest in the good brush, and keep it clean, because it's cheaper in the long run. Brush from wet to dry.