question

>> Some of my UE4 clients could not successfully login with their steam accounts, the logs indicate this: Unexpected HTTP status from Steam WebAPI GetPlayerSummaries: ServiceUnavailable.

I assume that is an error generated by LoginWithSteam API. I’ve found some info on Steam Status and Steam down? Current network status and problems | Downdetector. It shows that there is an outage on 8-10, so this issue may be related to it. Does this issue still persist?

>> Also some of the clients can login successfully but when pinging the regions, EastUS and NorthEU, they get very high pings from 3000 to 4000 and the matchmaking ticket is not created.

Does the latency you mention is the latency between players and Multiplayer Servers(MPS)? If so, since MPS supports to deploy server in different regions, you can choose low-latency regions for players based on their location. Just in case, you can refer to Quality-of-service (QoS) beacons to measure player latency to Azure - PlayFab | Microsoft Docs. And we need more info to investigate the issue that matchmaking ticket is not created, what is the error message when the Matchmaking Ticket was not created successfully?

>> Is the PlayFabClientAPI is having issues or responses with high latencies?

Currently, PlayFabClientAPI doesn’t have any issue. If you are encountering the high latency with PlayFabClientAPI, you can refer to latency is high,Send the API, and the latency is high - Playfab Community.