While I generally consider myself pretty handy, I`m of the opinion if it involves Natural Gas, Water, or Electricity, I`d feel better having a professional take care of it. Basically anything where there`s a risk of damage to the home.
Light duty/low risk stuff I`ll dip into like sink drains or replacing outlets; but stuff like water heaters or adding circuits to the electrical box is outside my comfort level.
The big thing I worry about with plumbing issues are the shut off valves. The last house I had was built in the 50`s, and every time I turned a (threaded style) shut off valve to service one thing, the dang thing would start leaking from the shaft of the valve. Now you`re fixing your original problem and replacing the shut off valve.
To JSFM35X`s point, there`s also that worry that whatever part you`re putting in is a cheap short lived part vs a quality part that lasted much longer in the past.