My first good quality bike had a 3 point fixing rear rack, and it was absolutely awful if used with loaded panniers. The rack would invariably start to sway slightly from side to side and the swaying was amplified by the action of pedalling. Partly this was due to the lack of triangular struts at the rear of the rack, but the single fixing at the brake calliper was a major factor.
Pretty much all the racks available nowadays are vastly superior, but I would still try and avoid using a 3 point fixing if I planned to carry heavy loads.
With regard to the risk of the seatstaty fixings of a 4 point rack fouling the brake calliper arms, Tubus struts are designed to be bent. Normally this would mean bending them down from the rack towards the seatstay, and possibly bending them inwards to bolt them to the inside face of a seatstay boss (instead of the outside face). However, you could probably just as easily bend them up towards the saddle, and similarly bend them outwards (p-clips can be rotated 180 degrees to put the clip in line with either the inside or the outside of the seatstay).
Tubus also publish the dimensions of their racks in pdf format on their website, so you could get a good idea of whether horizontal unbent struts would be likely to foul the brake calliper arms and therefore need to be bent out of the way.
Lastly, the straddle cable in the photograph looks far too high. Lowering it will improve the mechanical advantage of the brake.