I book about 240 flights per year (although not the US) and I have not yet had problems with any of the agents I have used from Skyscanner. I had the same problem as you, found bad reviews for almost every agent I used. What we have to remember is that people are much more likely to post bad reviews that good reviews. The ones I can think of off the top of my head that I have used without any problems are: edreams, ebookers, budgetair, gotogate, tripsta, lastminute.com, expedia, senturia
One thing to keep in mind is that booking through one of these agents means that you will likely have more restrictions, may not be able to change or cancel, or if you can change, the fees will generally be higher as they charge the airline change fee plus approx 25e. There also may be more restrictions on baggage or no baggage included on some flights so be sure to check and factor this in to your cost to determine true savings.