Just to play devil's advocate for a moment, you're faced with the job of replacing an old exterior panel. When you pull the old one, you find a hodge-podge of NM, MC, BX, etc. poking through one large hole through the old wood siding, all with sheaths too short to pull out enough to bring into the bottom of the panel to be covered later.
How do you compliantly land all of those conductors in the back wall of the panel?
First off, there should never be mc and bx going thru a large hole. They need to be connected to the panel.
There is never a good way to do this compliantly. IMO, I would leave it except for the bx and mc and fill the connector with some sealant.
Why would you want to go thru the bottom of the panel. An outdoor panel cannot have nm cable run to it if the nm is exposed.
I have argued this with Iwire many years ago. He insisted you could drill individual holes and bring them into the back of a panel. I would love to see someone install 30 circuits, with 30, 40 and 60 amp conductors on some, and get it into the back of a panel.
Imo, if you don't allow the installation thru the back then the outdoor panel is worthless. You could always install a JB on the back side of the panel and run individual wires thru a 2" conduit. That would work if you came out the back into a crawl area.... That just creates a tremendous amount of extra work