Are on-line business reviews from customers a trustworthy gauge of service? (apologies if this is a worn-out subject)

We all know that unhappy people are more likely to voice their feelings than happy people, right?
And what about cajoling friends to give glowing reviews? Attempts to sabotage good businesses for competitive or personal reasons? How about customers who enjoy superficial displays of consideration when another business simply sticks to being honest and doing work in a manner that is considerate to the employees as well as the ever-demanding customer?
Not to mention simply making up numbers of happy customers and their testimonials.

Maybe I'm mistaken but it feels too simple to imagine we should put trust into a wide-open system of reviews of dubious origin to determine whether a business should be considered viable or not. Very open to manipulation and abuse.
But maybe I'm wrong and it's a perfectly honest and acceptable way to evaluate businesses today.