Lots of complains, but people underestimate how much money, time and effort the company spends on a customer service. It's a war out there, traveloka raising half a billion dollars, "old" companies must fight back.
Not only their prices are usually the lowest, but they also offer matching prices.
The room was not as advertised? Booking would at least offer a coupon for next booking.
Hotel was overbooked? They would find you a room of no lower standard, in similar location, and they cover the difference. It happens that if there's a big event in the city, they might cover your 5* hotel if you booked a shitty 2* room.
Many small properties have shady businesses, booking.com protects you from this.
They might not use the shiniest tech, but their customer service (which is biggest portion of the work force) is top notch.
Source: shared bed with one of the cs agents