I've read people on other forums that use it, they just make sure to rip out the plastic windows obviously. I personally would stick to regular cardboard unless you couldn't get any.

i started using junk mail in i believe my millipede compost pile they don't seem to mind it. i just tear out all the plastic film stuff and thats it. MANY people on youtube use junk mail as bedding never heard anyone claiming it was actually bad to do yet alone warning people not to do this so i would assume it is safe.

i just added junk mail about a few days ago so i cannot honestly report on how it's going it will take a few weeks to compost it in general by the millipedes yet alone worms

The main trouble with paper is that it tends to wad up into dense clumps, even if carefully shredded to make it fluffy. Corrugated cardboard is less dense and you can just tear it up a bit by hand and toss it in. But there are folks on this forum who compost paper successfully. I wouldn't worry about the inks or bleached paper.

If you have recycling available I would let the junk mail sans plastic windows go into the recycling bin. The cardboard (not foam) egg cartons and similar creap type material formed coffee trays I would give to the worms and not to the recycler. If was a worm I think the egg carton or corrigated cardboard would be much tastier than junk mail, office paper, or cereal box board. The reason being the water holding ability of the fibers and the shape of the shreds holding more air than flat shredded office papper that mats and does not allow air in. The air allows bacteria to grow. The worms eat the bacteria and wee beasties. Yes a person can use junk mail or office paper or cereal box cardboard. But enough material probably comes into most houses, that the worms can only use so much.