Seastar is absolutely different approach, based on shared nothing architecture. It's not bad, but you cannot easy compare some async multi threaded architecture with it.

Seastar it's something like multi process architecture, where every process doesn't have synchronization, except n spsc queue per process (used to communicate between cores) and have only cooperative multitasking

So it good scales, if you don't have a lot of data to share, and have a very good work load balancer

But commonly you haven't, so go-like approach more and more easy