I view flwm in the same was as I view busybox.
Just as you, and likely many, have opted for coreutils to replace the shortcommings of busybox, the same holds true for the window manager, Just as busybox will never be as robust as coreutils, flwm, will likely never be as robust as the other much larger alternate window managers. Flwm is fully functional for intended use, just as busybox is.
My philosophy for Core is "just enought" X GUI Linux desktop and support infrastructure to enable you to pick and choose the applications, window manager, file manager, utilits.... that you want or require.