Mine were originally sized correctly and are in the process of being resized for lower flow temperatures. However, the individual timer/stats are not about avoiding correct sizing and balancing - they are to allow different rooms to be heated or not at different times. One bedroom is in use day and night, so needs heating, while the other two need no heat for most of the time. The bathroom needs heating before getting up in the morning and in the evening, but not during most of the day or in the night. The living room only needs heating in the evening during office/school days, but all day at weekends or WFH days. The kitchen only needs heating in the evening or weekends, as does the conservatory (kids computer room).
The point of individual timer/stats and valves is flexibility, with each room doing its own thing at its own time and adjustability, allowing the living room to be hotter than usual if someone is ill, without affecting the rest of the house.