He's referring to Ay lol. "Ay in atk, to maximize his round 2 burst (around 21k damage if he crits)"

Honestly, its about your playstyle preference. Also take into account where your Bp is at. Roshi is a monster if you help him survive. In order to do this though, you would have to be willing to 1 Most likely play Earth as a f2p(most people hate this main) and 2 have enough resources to put into him and have an extremely good set of gear. At around 50k or lower roshi will provide no benefit to you simply because he wont survive. I think 60k is where he's decent but still lacks survivability. You would also need a way to get his chase off to maximize his damage. He's just very and I mean VERY susceptible to burst damage. I'm at roughly 95k and my roshi will still die to lightning blitz against people 10k lower round 1. Usually happens if I'm not playing earth main though.

Ay on the 3rd on the other hand will most likely help you excel more pvp wise. Super armor, and depending on team composition can maximize his capabilities round 2. Like others have stated, he can one shot people or get close to it if you are needing a burst ninja. Some are arguing that you have to play lightning main but he works with earth much better in my opinion. Wind main works well with him as well but only after we get 4.0 which is probably when we'll be getting the treasure so it works out. Only downside to him is that he can be cc'd and debuffed due to lack of immunity. Providing feedback on him is a little difficult though simply because a lot of us don't have him yet. So our opinions stem from matches we've seen or gameplay of him on YouTube.

As far as end game meta's are concerned, Roshi is used more often without a doubt. Mostly see him paired with Kurama link on the CN side. Also we get more ninjas that are able to trigger Roshi's chase which becomes deadly round after round. The 3rd raikage is more difficult to place meta wise in CN simply because meta is always changing; as it should. I think you will still get a good use from the 3rd Rakage for a long time, I don't think I would consider him "end game though" although he's very close.