For nwell resistor design nature, the well resistor is not good.
As we know, the well resistor is formed by a indenpent well and two contact to form the metal connect.
Because the well implant is more deep, so there is lateral diffusion which is a main factor to do effect the accuracy of well resistor.
In the mean while, the different drop voltage between terminals of resistor results in a different resistor value . this is called voltage coefficient of resistor.
Due to the two disadvantge, i think if you have a matching requriement , other type resistor can be selected for improving performance of circuit.