I'm kind of on all sides at once
1. Many platforms have Human Design Guidelines. Those of macOS and Windows are thousands of pages in total. They define ergonomic interfaces. The (imho, and following those guidelines) huge advantage of sticking to them is making it easier for the user. Windows users are used to programs designed following Microsofts Design Guidelines, macOS users are used to following Apple Human Interface Design Guidelines. Everything that moves away from these standards makes the UI less intuitive.
2. If you ask a designer, they've got their own special set of rules and want a program to stand out because it looks unique. Good designers might make this work without losing the intuitive approach to the program, bad don't. And programmers aren't designers.
3. I think there are reasons to create different frames for different widgets even if following route 1. Or better, for different OS. While the controls are native, the design guidelines for each platform might define different placements, be it (imaginary examples) dialog button order, dialog button position left/right, use of tabbed interfaces, etc.
4. In this example, adjusting the autosize property of the TToolButton and checking transparency of the groups would probably be my first approach. To be absolutely correct, check the design guidelines of each platforms to check whether all buttons should be autosized independently, or as a group with a fixed common width.