It's probably something you can optimize later on, if you actually hit the scale you're concerned about? There are a lot of similar services too, at different price points. At the end of the day you're probably paying someone to send emails anyway, unless you really want to run your own SMTP too (generally not a good idea). It's a lot easier if it's just all one provider.
The main thing is handling subscriptions and unsubscribes. If you have to manually sync them (and opt-in), it's more complex, and if you don't do it well, that will impact your deliverability (i.e. if enough people mark you as spam because you don't make it easy for them to unsub, over time your domain will get treated more and more like a spammer).
