There are many issues with the way emails are rendered in the current version of Outlook desktop which causes the finished email to look less polished than it does in the email builder.
The corner radius of buttons is ignored.
If I place a background image the padding (at least of images and buttons) is ignored.
Sometimes when I have used a black background and white text, the text received is also black making it invisible to see.
Line height increments are ignored. Only seems to be whole numbers that are rendered correctly so spacing ends up too close or too distant.