Slight correction: by default Debian/Ubuntu put all modules potentially required for initial boot into the initramfs. That's still a very small percentage of all modules. I.e. you only need those modules that will get you so far into the boot process that you have access to the root partition with all the remaining modules.
E.g. if you want to do network-boot over wifi, you'll have to add a initramfs-hook script to add the wifi modules for your hardware into the initramfs [1]. They are not included by default.
You're right, I misinterpreted what "most" meant in the mkinitramfs config. Interesting. I've not seen any difficulties with porting Ubuntu between different hardware configurations, so it seems to include a reasonable amount of them.
Every now and then I'm tempted to try "dep" instead of "most", but then I realise there just isn't enough benefit!
E.g. if you want to do network-boot over wifi, you'll have to add a initramfs-hook script to add the wifi modules for your hardware into the initramfs [1]. They are not included by default.
[1] http://www.marcfargas.com/posts/enable-wireless-debian-initr...