It is easy to deal with an unpredictable supply, it does require regular analysis of water samples for a time, but when you factor in the running costs of an RO unit let alone the purchase price, that outlay is reasonable.
My alkalinity varies between 16 and 135 and calcium between 20 and 70. All I do is use a alkalinity test and a TCS reading to allow me to make a very educated guess at what today's brewing water contains. . . . Oddly enough it actually agrees with the same water sent for analysis pretty closely.
Frankly if you can taste the difference between water containing 150mBar of sulphate and one with 200 your taste buds are way better than mine