I think there is two differing philosophies between the two as well. StartAllBack is designed to be retro. It strictly focuses on bringing past items to Windows, i.e. windows 7 start menu and windows 10 taskbar, even it's themes are retro. Start 11 is more forward thinking. It is designed to take Windows 11's strengths into account while adding features for power users. Start 11's taskbar by default barrows parts of Windows 11's native taskbar while inserting a few new features. This means changes in Windows 11's design will carry over.
I think, maybe, what you are experiencing is the fact Start 11's taskbar is almost brand new. In fact, I am not sure it is even 100 percent finished yet. I am sure Neil is still taking request's of features people want added in.
As for theme's and skinning, I believe this only recently became possible thanks in part to 22h2's design changes. Windowblinds 11 is now in development and this will be Stardocks way of skinning Windows 11. I will also note, stardock has a different philosophy with skinning. i.e. keeping Windows 11 features in tact rather then retro fitting Windows 10 skins onto Windows 11. StartAllBack has also had a head start with skinning because their program is about the same as it was during the Windows 10 era. Start 11 on the other hand is brand new.
Once WindowBlinds 11 drops, and that will be soon, the whole custom skins landscape will change a great bit. I believe it will be worth the wait.