Anything with a taproot is like that- best transplanted early and small, before the taproot is too long. Here's an old pictrue of a wintersown lupine that shows how big a taproot can get, and how soon. As you can see the taproot is already longer than the seedling is tall.
As long as the gardener gives the root the respect it deserves, seedlings can be transplanted. It's not that difficult to remove them from nice fluffy potting mix. Later, after they are established in garden soil and the taproot is 15" long, transplanting is a whole other story.
Karen
