@impeeza, I used nsz_v4.1.0_win64_portable to compress an NSP to NSZ using these settings:
Now using DBI to install the compressed nsz it installs fine and plays. I then tried installing with tinwoo/atmoxl/tinfoil and all of these failed. I could install OK, so I dumped the installed nsp with nxdumptool and then extracted the nsp on my computer. What I found is that the main nca file is corrupt, so that's why it doesn't work - the header is fine but the sections are all corrupt. Also when you install with dbi it takes far longer to install, probably because this works (extracts the NCA) properly.
I think it's down to the nca_writer.cpp file, processChunk function or maybe the write function, so that code will need modded to be compatible. I then tried using Tinfoil to install some nsp's from a shop - I think these must have also been nsz files that were downloaded as these also failed to play even though they installed - down to corruption in the when extracting.
So the bottom line is to use DBI installer as that works great, maybe this will get fixed at some point in tinwwo/atmosxl but you'll need to wait for now.