The secret is 4" of soil
I have been doing this for many years and now realize the best success is with containers where you can have 4" of Pro-Mix or seed starter. Anything less is a problem because you have to constantly monitor the moisture in the container.
4" of soil is easy if you are using milk jugs. The salad containers allow for 4" of soil but aren't deep enough to allow for the seedlings to grow without removing the top. When you remove the tops, you have to worry about keeping them moist.
Just my thought here.