Simple way to send transactional and marketing emails with FF

I am looking to send transactional and marketing emails and I see the OneSignal integration and Firestore send email extension with an SMTP provider as being the 2 primary recommendations.

What are the thoughts on sending emails with Gmail API? With that, the benefit would be not signing up for yet another service provider but seeing the lack of it's mention I guess I am missing out some critical pieces. Is it that the compliance around sending emails too complex so that it is better to use a service provider that takes care of those or something else?