Thoughts from a layover: ICN has a huge number of lounges—every wing has two. Almost all of them are accessible with the PP from CSR; at most, some require flying Star Alliance, like Asiana, but as a Bay Area worker, flying UA is already routine.
The lounges at ICN are all high quality, and because the terminal's commercial offerings are so strong—lots of luxury brands, very comprehensive—a good number of people decide to go shopping instead of crowding into the lounges, so the lounges are quite comfortable.
Due to time constraints, lz only visited Matina and Asiana. Both have showers, and there are also massage chairs. A few photos attached.




In reality, this comes down to ICN's hub strategy at work. Incheon Airport has long aimed to establish itself as a transfer hub for Northeast Asia, using ample commercial amenities to keep transit passengers inside the terminal and spending. The abundance of lounges, combined with Priority Pass access, is to some extent the result of the airport and airlines competing for transit passengers. If the passenger flow structure changes someday, or Star Alliance tightens its access rules, this level of comfort may not last over the long term.