Consumer and rental carpet cleaning machines typically run at lower water temperature and weaker suction than commercial equipment, both of which directly affect how much dirt actually gets lifted out versus just pushed around and left to resettle as the carpet dries.
Weaker extraction also means more moisture stays in the carpet afterward, which extends drying time and increases the risk of mould or mildew developing in the underlay, especially in poorly ventilated rooms.
For light, regular maintenance between professional cleans, a rental machine can be a reasonable supplement. For an actual deep clean, particularly ahead of an inspection or after a stain has properly set in, the gap in results between DIY and commercial equipment is usually obvious once you see both side by side.
