If the service you're embedding has a pop-up ( lightbox-style ) version of their embed, that will usually work best on mobile.
The reason is simple.
Mobile displays are small, and those widgets need every possible pixel in order to display their forms and UIs. If you have even a few pixels of margin around an IFRAME embed, that can break everything.
Using breakpoints, you can create both inline and pop-up versions of the same widget in your website.
Create one that's inline, for your desktop & tablet users, and one that's a pop-up for mobile landscape & portrait users.
