They’re still fine for gaming, just not as fast as the fastest Intel processors.
Games generally heavily rely on a single main game thread, so single-core performance is still the most important factor for the gaming performance of a CPU. AMD has not quite matched the IPC (instructions per clock) of Intel yet, though they’ve made major strides in that regard, and the Ryzen 7 processors are clocked relatively low because of their high core count. That all equates to lower single-core performance on the 1700/1800 Ryzen processors compared to something like the 7700K from Intel, which results in lesser gaming performance when the CPU is the bottleneck (typically at resolutions of 1080p and below). However, once you reduce the CPU bottleneck by jumping to 1440p resolution or higher, the difference between the 1800X and 7700K becomes much less pronounced. Also, we have yet to see what the quad-core and hexa-core Ryzen 3 and Ryzen 5 processors will look like, especially in terms of clock speed. It’s likely those will be clocked higher, resulting in better gaming scores.
Anyway, the general consensus is that the 1800X is best for content creators who rely heavily on multi-threaded tasks. So video encoding/streaming, 3D rendering, or code compilation and light baking with UE4, those are all good reasons to own an 8-core/16-thread processor. And AMD is now providing great value for those users with their Ryzen chips. Besides that, they also perform very respectably in games, albeit not top of the line.
In other words, there’s a difference between “not good” and “not the best”. People often seems to mix those two up.