not true at all for most alternative brands (they are not knock offs, the patents are expired so they are legal, and comparable in quality), same for cloned sets (shady companies cloning lego sets using alternative bricks (the bricks are legal, the cloned sets aren't). the quality of alternative bricks is good. the quality of the instructions as well.

For me it is 1. Terrible quality of all rubbery/soft elements. 2. If it is original model (instead of ripping of existing set), it often contains huge, shell like elements, that can't be easily be in custom designs. 3. I guess the previous point doesn't really matter, when bricks are designed to be assembled once and are impossible to pull apart without hurting your fingers.

The 2. is very annoying. Especially when big sets fall apart due to this issue.

Let me add this: 4. no spare parts available. So when I break weird Chinese invention the whole set becomes useless without that very special part. It happened few times and I got back to used Lego sets.