Don't know. Honestly, a phone is a phone. If it calls for you, messages for you, allows you to watch videos at 720p/1080p and does this with a decent battery life you should be good. I got my S5 in 2014, replaced the battery in 2016, and it is still good to go. Like brand new, pretty good battery life.
After iPhone 7/S7, everything else is mostly a gimmick. Nice, new camera, great. But I do not see much use to upgrade.
I read the S9 coverage recently, and noted that one of the individuals said that it has gotten to the point that upgrades every year are not too significant. I think the S9 is just the S8 with a better Bixby and camera, but thats about it. Performance wise, it should be the same.
It's issues would probably be the same as those of the S8 when it came out.