Submit external forms data to Marketo

I always advocate for using Marketo form submission in the background (or, as I like to call it, the "Ghost form approach") over the "formSubmit" REST API for making form submissions to Marketo from external forms.
 
The former is a lot more scalable, agile, and clean than the latter. While the latter consumes API calls from your quota, the former doesn't!

There are certain eccentric use cases where the "formSubmit" endpoint is a bit more suitable from an implementation perspective, but overall, I generally prefer the "Ghost form approach" over the "formSubmit" API endpoint.
 
Additionally, when it comes to upsetting data from the external forms, I see a significant number of people using the vanilla "leads" API endpoint, which is way worse than the "formSubmit" endpoint, as you don't get any filled-out form activity in Marketo.

You may be okay with that for now, but in the long run, it cripples your ability to use the powerful and widely used filled-out form activity in your campaigns.
 
Do you use external forms in conjunction with Marketo? If so, how do you bring external form data into Marketo?