Obviously if you stick to one key you can't go far wrong harmonically, but without some extra sense of purpose or direction you're unlikely to come out with anything compelling in terms of tension and resolution or melodic flow either.
In my opinion, you CAN stick to one key and create a sense of purpose and/or direction. But it does mean you have to be more intention about what chords where, your instrumentation (and it's evolution), as well as the voicings you picked. Then add in articulations and dynamics and you have so much to pull from. Something that Billy Joel does, and he learned it from the masters of classical music I believe, is a inversion chord cadence vs. a root position cadence. Such a simple, small change but it can really make a difference between a somewhat firm and very solid cadence in a song.