I installed FMCboot on my friend's console and I tried playing from USB, it wasn't really playable (except few littles games which don't require excessive disc access, like katamary).
I then tried ESR + patched ISO burned to DVD-R disc, and it works beautifully
it prevents you from buying the network adapter and all the needed hardware.
I don't know why burned discs are not a recommended solution. Everyone recommend network adapter, but I prefer Disc based gaming. it's easy to use, insert disc, it auto-launch.
it has a 90% compatibility.
If you want a disc free solution, then network adapter is the way to go.
To install FreeMcBoot, I didn't find a lot of information.
like "will my memory card be deleted/formated?" no, you just need enough free space to install it.
"is the network interface required to play from HDD? can't it be accessed directly from internal HDD?" never got that answer.
All install guides asked for a way to run homebrew on a non hacked console (using Action replay, or swap trick etc.), but they didn't explain that you could use a modchip console, that's the easiest way and what I used.
Another way is to ask someone with an already hacked console to install FMcB on your memory card. there is a thread here with users willing to do it for you.
FreeMcBoot is a program installed on the memory card location used at launch by the PS2. it's autobooted if the card is inserted and displays a menu (you can choose which item the menu displays and which programs it redirects to).
the default package wasn't at my taste so I edited it to install more apps to the memory card.
The install process was easy and went smoothly.
if you want precision on the install process, or the way to make your own menu, let me know.
I can't help with Network interface/HDD.