Expat, you're right in saying that calculating alcohol consumption in "units" is a UK thing. However, in the US they do the same thing, and have more-or-less identical recommended limits, but under the name of "standard drinks".
SGB, the recommended limits aren't totally made up; there have been long-term alcohol consumption studies that demonstrate increased risk of disease the more people drink. Sure, they don't take into account variations across the population but the idea is to make it simple enough for people to easily understand.
And I disagree about your definition of what constitutes a problem. By your standards, my ex didn't have a problem as she didn't feel particularly rough most mornings, she managed to make it into work every day before she got made redundant, and we could afford it as I had a good-paying job. So no problem, right?
Away from all that, though, her drinking caused immense problems in our relationship (and for our children) as she becomes aggressive and volatile when drunk. She didn't feel particularly rough when she woke up because she was used to it. I think she'd largely forgotten what waking up properly sober actually felt like. She could last a day or two without a drink but couldn't reliably control it once she started drinking.
More to the point, she undoubtedly had a significant drink problem back then when she was doing a bottle of wine a night for years. And she's a self-confessed alcoholic today.