We're currently implementing a solution where we would like to avoid being Throttled. We decided to use a Service Account because our solution consists of write-operations to Taxonomy which are not supported when using App-only permissions.
Do we run the risk of getting throttled faster when making use of a Service Account compared to making use of App-credentials?
The Guidance in the article below seems to suggest throttling behavior is different when using App-crredentials compared to a Service Account ("normal" user). As described in the article we are filling UserAgent for each CSOM query being executed but the article also recommends to register and use AppID/Title whenever possible (which are involved automatically I guess when running in a Add-in context)
As noted in the documentation, using app ID is recommended where suitable. This does not, however, mean that you can't use also service accounts, as having an app ID registered does not mean that you'd be running in app-only mode.
We do not have any specifics on the detailed differences or impact of having app id or not