Most of my custom is Aga / Rayburn appliances, the casting will last a life time, the burners parts on all fuels will need to be changed from time to time like most appliances but regular service is a must.
Gas models will normally give you no trouble as like anything that has been installed and commission properly, the t/couple and pilot burners tend to be the most replaced parts.
Some Oil models have a higher component rate failure but we tend to find these are mostly the ones that have not been installed/commissioned properly.
The oil models that most people hate to work on are 368k,480k mk1, 480k mk2 the newer 480k mx series are better than the earlier models but still have higher failure rate than Gas models.