Shared IP
A sending IP used by many customers of a provider.
Most senders start on a shared IP pool. Reputation is collective: one bad sender can drag the pool down, but established providers actively manage their pools. Below 100k/month, shared IPs are usually preferable to dedicated.
reading this as teams choosing a transactional API
Judged purely as an API. What the request contract guarantees on retry, how complete the webhook event coverage is, whether suppressions are readable and writable programmatically, and how much the message log retains when something needs debugging in production.
Applied to shared ip, that means weighing idempotency keys, webhook coverage, event stream, and operating track record ahead of the rest, against high-frequency programmatic sends where a missed delivery is a product failure.