Why is package x not available for myisa but another for ArchPOWER?
Some packages aren't supported on some ISAs and thus can't be packaged. If you feel up for it, provide issues and patches to the respective projects and help them out!
Popular examples:
firefox
nodejs
chromium (and derived projects)
Why does an architecture independent any package have broken dependencies?
For ArchPOWER any (architecture independent) packages are hosted separately so all 4 supported ISAs may utilize them, it's an efficiency thing.
As per pacman.conf it looks like this:
[base-any]
Server = https://repo.archlinuxpower.org/base/any
[base]
Server = https://repo.archlinuxpower.org/base/$arch
By design, this may cause issues should any of those packages depend on a architecture specific binary package which isn't available for your ISA and failures should be examined per case.
Example:
vimb -> qt5-webengine
vimb is theoretically installable but qt5-webengine is not avaiable on anything but powerpc64le due to the non-portable nature of its code. This is a natural downside of vendors not developing with portability in mind.