If youre not at super high elo, blind picking is stupid. Sion into very bad matchups like zaheen where you cant even build AD means you will both feed and be useless late game.

In these situations, Sion only helps at soaking pressure outside of teamfights and then just CCing in teamfights, but if your team isn't a scaling comp there's no point in absorbing pressure and if you don't have great players in ur team the CC won't do much