Likely, the rx 5500 is ROP and bandwidth starved compared to the typical Navi architecture. While ROPS and Shaders got cut in half, shaders did not and are more likely to be more strained for resources.
Considering Navi was already strained for bandwidth considering overclocking results being disappointing because of the lack of overclocking headroom in the memory, the rx 5500 is likely showing those some bottleneck limitations.
The GTX 1660 on the other hand while having slightly worse bandwidth ratio compared to a rtx 2080, has excessive ROPs(64 vs 48). Considering Nvidia's architecture has typically done better compared to AMD with less memory bandwidth, it should not surprise people when a gtx 1660 is faster than a rx 5500. Lets not forget the GTX 1660 has an additional 2gb of memory as well. 4gb of memory is beginning to show strain at this point.
The RX 5500 Navi is AMD's ambitious attempt to disrupt the sub-$200 graphics card market just as the RX 5700 series did to the sub-$400 market, bringing in much needed competition. It won't be easy as NVIDIA tried to preempt it with the GTX 1650 Super and GTX 1660 Super.
www.techpowerup.com
RX 5500 review is up.