You can do all the things you've listed. But you can't build an arbitrarily complex DAG of tasks. Instead, you must order everything into a sequential list of 'stages'. Within a stage you have a list of jobs, which are run in parallel. This might be enough for your needs, but I'd like flexibility to express dependencies for each and every task, Say `t1 -> ((t2,t3),t4 -> t5) -> t6`, etc.
And it's a pain to propagate variables from one task to another, like say setting an environment variable in one task, and getting its value in a downstream task. You end up writing scripts to write them to a file, telling gitlab to make that file an 'artifact', and then reading it downstream.
And then there's the transient errors, all of which are listed on open issues. We're using the hosted gitlab.com, and on some of our CI pipelines, we have a dozen or some stages. The chances of a transient error, .e.g. a runner getting a 503 from gitlab.com, are pretty high.