1. Student paints aren't always pure. They may be labelled as 'hue' which means made to look like a pigment, but not made of that pigment. One common example is Cerulean Hue which is not made up of cerulean pigment but pthalo and white. These mixes may make a muddier, (less pure), colour or not mix how you expect them to as the properties of the other pigments, such as the white, will affect the mix.
2. Student paints can contain a shorter-lasting fixative or binding agent. They may degrade over time or even go mouldy! Some less expensive acrylics also get very plasticky, and can peel off the canvas or paper.
3. Some papers, particularly watercolour papers, contain so much rag that they absorb the paint really quickly. They don't hold detail as well or give you time to work your painting. They are also unforgiving, and will pil, (degrade), with the smallest touch. They may also need so much more paint that they become a false economy, (in other words, you save on the paper but you pay more than you saved on the paint).
4. Some papers are not pH neutral or acid-free, meaning they can discolour or leach color over time.
5. Some paintbrushes lose bristles as you work, which get left in the paint. Others come apart or rust at the ferrule or the wood swells or flakes as they age, which can also contaminate the paint.
6. Some cheaper primed canvasses aren't well folded or tight enough. If not stored properly, they can get pressure marks which can't be easily removed, (or can't be fixed at all). The frame may be badly cut, loose or uneven. Primed canvasses may not be pH neutral.
5. Cheap materials can make you work harder to accomodate them, taking longer to complete works and making the learning process that much more complex. Moreover, if you get frustrated with the problems that arise, they can stop you enjoying your art as much as you could!