I had 2 shoutouts for my main channel. They're bigger channels than mine but not more than 3k subs on their channel.
From my experience, does it help? So so, its a bit like leaving a flyer on some person's shop saying, "hey there's a channel out there that seems decent, check them out". So if someone saw it and is interested, they'd visit the channel if not, then nothing. So I think it might help in the long run as the "flyer" would be there for a long period of time.
On the day the video was published, I got roughly around 3 subs from 1 shoutout and 8-10 subs from the other(as you can see, one was significantly bigger than the other channel). With this, I say its ok, it's not necessary but it just helps spread the word about your channel a bit. That's my experience.
I saw a channel got a collab with someone that has 18k subs but has an average of around 2k views per video. Well, from what I see, they probably had the same amount of subs that I got from my shoutout.
So I have to say, if you want to collab and get something out of it, choose better. If you just want to collab because its fun, then do whatever because collaborating I think helps a bit with SEO. I think it helps because now when I search for my name, their video is in my search results and vice versa.
