Will journals accept a meta-analysis of case series or observational studies? What if every study is single-arm?

Yes, you can.

In medicine, not every specialty puts the same weight on randomized controlled trials. Often a new treatment is developed, judged effective right away, and used widely. This is not unusual.

The value of a meta-analysis depends on the kind of studies it includes. If your field has both randomized trials and observational studies, a meta-analysis of the randomized trials will of course be considered more valuable. But if your field simply has no randomized trials, a meta-analysis of observational studies is already quite valuable.

Further, if you can choose between studies with a control group and studies without one, a meta-analysis of the controlled studies is of course more valuable than one of the uncontrolled studies.

But if single-arm studies without a control group are the only information available in your field, pooling them to understand the real value of a new operation is still very meaningful.

In most cases, these studies are single-arm because before the treatment existed, patients could hardly survive. Once the new treatment appeared, any reasonable person would find it hard to justify asking patients to be randomized when half of them might then not survive. In that situation, simply pooling the single-arm studies to show the overall benefit of the treatment is quite meaningful.

A meta-analysis of randomized trials is easier to publish, but when we choose a topic, we still want it to be related to our own specialty. If your field has only single-arm case series, organizing them and publishing before anyone else does is an academic breakthrough in its own right, and worth publishing.