Maybe the biggest drawback for tritium is the decay energy of 0.02 MeV compared to 5 MeV of Pu238. On longer missions also half-life may create problems (12 years / 70 years).
That's a good point. Of course if you can produce tritium cheap enough it might be feasible to just start each mission with a grossly over-powered nuclear battery.
E: I'm very slow with my phone...