Yeah this is pretty much right. If you don't get him scaled up with chakra in round 1 his damage in round 2 isn't that good but once you get his mystery spam going it's really strong. So yeah, sometimes the damage can be underwhelming if you don't have a team that generates chakra to scale him but if you do he's really great. For example, a team like this is well suited to Sasuke:

Like the other poster said, he needs the good supports (Nagato and Itachi are best with their BT too - or at least BT on Itachi's mystery so that it becomes prompt/immediate and costs 0 chakra, and Nagato's Y+1 Rinnegan immunity passive and Y+1 debuff reflect passive) but when you put him in a good team he will shine. So like, here, you can set up Itachi's barrier round 1 and then it will consistently give you 100 chakra for the next few rounds unless it gets overriden (but you can play smart and avoid that). Using Earth main's mystery will give you 40 chakra in round 1, and you also have the chance to get more with Nagato's standard that drains chakra. Then going into round 2, you will have a nice scale from Sasuke absorbing the previous round's chakra, and because of Itachi's barrier his cooldown will be 0. By round 3, he should be doing a lot of damage. And, if you keep shields on him he'll be immune to debuffs thanks to Nagato which is really nice.

Also...in terms of whether you should use the fear passive or the chaos one. I've tried them both, and personally I think chaos is better. At first, I thought fear was going to be super great and OP but in my experience it's really not. Sure it goes through immunity and prevents the attacker from getting shields or doing any standard attacks, but I've found that by the time the debuff gets applied to them, they've already done their standard so it doesn't matter, and most ninjas only do one standard.

Like, if you're fighting Kakuzu and he gets feared on the first standard attack he does, sure he can't do any more standards but he can still mystery, etc. Under chaos he can't do any of that. Chaosing a bunch of people on a team is really satisfying. Yeah it can be cleansed but often it isn't. Plus with the Chaos passive you absorb chakra and chakra is the most important thing for Sasuke teams.

Fear does have its place in usage, in theory it's good against ninjas like Ronin Naruto and Summer Kakashi who rely on their standard attacks to scale. But imo, Ronin is too uncommon right now to warrant it and chaos could also stop him. With Summer Kakashi...sure, but I see Summer Kakashi with Edo Minato so often and in my experience it's just really hard to win against Edo Minato with Rinneasuke even with BT. The elemental matchup is bad, Minato has all those dodges, etc.

I guess if the people in your server cluster are running more Summer Kakashis and Ronins than Kakuzus, go with fear. In my personal experience it was underwhelming. But, try both out and see which you prefer.

Good luck!