A well conceived painting is the orchestration of many things. There must be a unity of the elements beyond the depiction of the idea for its success.
To help do this I recommend always using a unified approach. If you work outside, then finish your paintings outside. If you work in the studio, complete your paintings in the studio. The amount of time spent on the painting isn’t important; some paintings do not lend themselves to a quick alla prima approach. If you need to return to a spot to finish your work outside then do so. The approach you take isn’t as important as not mixing approaches.
You can never match the effect of outdoor light inside. There is nothing wrong with studio work; I paint in my studio all the time from photos and studies. My studio work has the same unity as my outdoor work because of the fidelity towards unity of implementation. If I need to correct a field painting in the studio I will set up a new canvas and paint it from the beginning. Whenever you preserve the unity of process your paintings will be stronger for it. I believe anything that fractures the process of painting diminishes the quality of the finished work.
After a unified approach, keeping consistency between the various elements is vital. There are many skills that go into the successful completion of a work of art. These skills must stay in constant harmony throughout. You cannot separate process from outcome. Outcome is the direct result of process and the ensemble of abilities you need for painting must compliment, not compete with each other.
Great paintings are not just a good concept; they are also a good performance. The ability to handle your materials effectively and the knowledge required for their use in service of the idea are as important as the idea itself. This is what is meant by the unity of a painting and this is what ultimately elevates it beyond the mediocre.