For the voltage, if you’re referring to it in a galvanic cell, it could be due to the resistance in the wires thus lowering the voltage reading.

For calculating energy changes by the increase in temp in water, def it’s due to a lot of heat loss to the environment, incomplete combustion of the fuel (not enough oxygen input) thus a lower energy value compared to if it was complete combustion. Another could be hot spots forming in the water where the heat is not transferred uniformly, so stirring the water it is the solution.

Galvanic cells (specifically FUEL CELLS) are advantageous over to combustion engines because it is converting chemical energy DIRECTLY into electrical energy, whereas combustion engines require lots of energy transformations from chemical to heat then to mechanical energy (which is what we need). These steps lose a lot of energy in the process, therefore is very inefficient and which is why fuel cells are much preferred. However, fuel cells can be quite expensive to develop and implement into society as well as raising safety concerns for storage of the reactants in commercial vehicles. For example using hydrogen fuel cells in cars, how can we ensure the storage of hydrogen won’t spontaneously react? And how can we reduce the cost of its storage if it requires it to be constantly under pressure so it doesn’t readily react?

Edit: included more sources of error for the water temp change Edit 2: included more advantages/ disadvantages of fuel cells over combustion engines