The vast majority of time, it is better to start seeds either indoors, or in a smaller container with the intention of transplanting the seedling to a larger pot. This is because it is really hard to get plants started from seed into the right spot in a mixed container if the seeds are sown directly into the container. That being said, there are a few plants that I think you should always directly sow into their first and only pot, and there is a simple way to remember which seeds to directly sow and which you should transplant.
Some plants develop a strong, central root, while others develop a network of roots that look like a rat’s nest of thread. Plants that develop a central root really don’t like that root to be disturbed, and those are the plants that you should sow directly. In the context of container gardening, the plants that should not be transplanted are almost always edibles. And it’s easy to tell which edibles have a single major root, because they are the plants with an underground part that we like to eat (i.e. carrots, radishes, etc).
What happens if you do decide to transplant a vegetable whose root you intend to eat? Well, if you damage the root during transplanting (which is very easy to do), the resulting vegetable will look like the carrot on the right. It’s also important to note that if you use potting soil that has chunks of wood in it (as many cheaper potting mixes are wont to have) you might have the same problem.
Oh, and FYI, even though this falls outside of the rule of thumb I outlined above, I know that poppies don’t like to be transplanted either. And like every rule, there is an exception: beets seem to do pretty well with transplanting.