Over time, organic matter will find it's way on top of the landscape fabric. This will then provide a media for weed growth on top of and through the fabric. As most landscape fabrics are covered with inorganic or organic mulches, weeds will also grow into these mulches. As many weeds are perennial in nature, the entire root must be removed to eliminate regrowth of the weed. This becomes difficult when the roots are growing into the fabric and mulch. If you thought weeding was tedious before, you will be pulling your hair out once you have a tangled mess of weeds, landscape fabric, and mulch.
In addition, your garden plant roots may also become tangled into the fabric. Any attempt to remove the fabric can damage the roots. If you like to split your perennials and occasionally move plants around, this task becomes much more difficult.
Most importantly, landscape fabric interferes with the movement of gases (such as carbon dioxide and oxygen) between the soil and atmosphere. This results in death of micro-organisms, plant roots, and animals living in the soil. I have worked on several gardens where the majority of plants were either dead or struggling from the use of landscape fabrics. All the fabric had to be removed, the mulch replaced, and the entire garden replanted with new plants. An expensive lesson learned the hard way.
Landscape fabric will degrade with time, especially when exposed to sunlight. It is not a permanent solution. The best solution for your garden is an organic mulch with no landscape fabric. See my blog article on Benefits of Mulches.