We've been working with google cloud build for a while now in my team, and what initially seemed like such a succint and easy to get started with build tool has caused us probably much more head ache and annoyances than a managed jenkins.
But that is just about it. Cloud Build works great if the only thing you want to do is build an image.
If you want to do any other stuff, lets say, filter some steps depending on which branch that is building, then stuff gets really complicated.
Either you have to roll out your own images for building that checks wether the branch is a certain branch or not, alternatively you can run your docker file by passing some shell script to it.
Either way your build file will be messy or you will have to maintain a bunch of images.
Not to mention if you're working with let's say maven and have many dependencies (Spring anyone?), you will have to download all of your dependencies for every build(!).
It feels like cloud build is an afterthought, that is not receiving a lot of love from google.
My tip is to stay away, use something else, and you know what? It's probably cheaper to pay for it, rather than having someone maintain it.