But for a current project I am trying to build something entirely on top of NetSuite without any iPaaS / middleware and so far seems to working out quite well

that's good to know, I'm a one man team here and I don't have much technical experience with integrations. So I was thinking Celigo might be a better fit since it seems Boomi requires more in house resources that we don't have at the moment

100% if Celigo covers your requirements it is much easier to administer. They have a free tier with one integration flow so you can play around for yourself and see.