Yes, you are right. That's the trade off. There's no 100% reliability.
I think you're still being overly optimistic. In some cases, there's 0% reliability. Just remember, if I cancel the default submit event + stop propagation that has no bearing at all on whether I separately submit the data using Ajax.
That's the thing about Ajaxified forms: even if I use the FormData API it's still not the same as the underlying <FORM>. FormData + XHR isn't supposed to fire HTML events. So making sure that a tool like GTM functions in all permutations might involve writing custom JS in itself, making way more work than simply using the provided onSuccess.
I just recently completed a project where I integrated Marketo and Google Analytics. I published an article and accompanying cheat sheet, both are available from Google Analytics-Marketo End-to-End Integration - I cover the form tracking extensively - you will get the step-by-step instructions on how to do it.
Hi Stephane
I have tried a couple of times to download the cheat sheet on Google Analytics-Marketo End-to-End Integration but it never came through to my inbox!
Lee
Hi Lee - some people reported an issue with the form subscription - being a freelancer, I couldn't afford to use Marketo to do it...
But if you email me at shamel@immeria.net I will send it directly to you.