I sow almost all of my plants in trays as starters. Any time of the year. And I also like to reuse things when I can. So for some seeds I reuse egg containers to start the seeds. The containers are not very deep, so are either best for late season starters which can go strait from the starter tray to the garden, or for seeds where I will eventually move the seedlings to larger trays. So, understanding those givens, and working within those limits, they really make for great starter trays.
I prefer Styrofoam and plastic egg holders. Not that eco-friendly till I can recycle them. But I am reusing them and multiprocessing them so, still one of the three pillars of “Reduce, Reuse and Recycle”.
Besides…. I find that cardboard “pots” in any way simply sucks away moisture from the compost I use for my starters, resulting in quite a few die offs. So I tend to avoid cardboard as starters. But that is just me.
As an example in the images below, egg containers with 10 cells each, with multisown radish. Average of 4 radish plants per cell. You are looking at well over 200 radish plants in 6 egg containers. And despite the shallow draft of the containers, the radish seedling look quite healthy.