I know this question has been asked many times.
I signed up for a new network and they have some SOI campaigns on there, with what I consider are very good landers of their own.
Do you still recommend using my own lander in front of theirs if they are already creating a good angle to get people to sign up?
Would direct linking be acceptable in this situation, or should I still use my own lander in front of it?
Thanks.
I would say going for your own landers. 
First, because you want to keep track of your Landing Page CTRs for better optimization.
Second, because you can improve your lander and test more angles and lander variations.
Usually every offer that comes with a pre-lander comes with a version without it. In case it is not visible in the platform, your AM can help on providing the deep link of that offer.