According to my play experiences, Erufuun's been pretty one-dimensional so far. I do think it'll be OU at the end of the day, but he has a couple of obvious flaws. First and foremost, the inability to actually attack and do damage are always, always going to be big problems for him. You only get so much HP to stall out, and if your opponent is constantly on the offense, you might not get a chance to set anything up.
Secondly, you need prediction skills off the chart to keep him consistently effective. I've seen this one stall team that had him maxed in defenses and when he tried to Encore my setup sweeper, I just switched out right away. Next time it came in on the setup, I outpredicted my opponent again and attacked right away, leaving the rest of his team vulnerable to a Zoro sweep (I love saying that). Godspeed if my switch-in happened to be Shandera.
I might end up trying him myself one day, just to see what the fuss is all about, but I doubt I could do much with him anyway since stall isn't my style.