The first thing about "cheap knockoff" that make them seem not as bad as might sounds is the fact that they are producing and selling dolls for a cheaper price. It's not the "china copycat" kind of knockoff where you get a blowdoll instead of a TPE doll. It's closer to the kind of knockoff as what you find in food : known brand which controls all the ingredients or the unknown brand that has 40% of the daily value of sodium per portion, but cost 1/2 of the price. Both taste good, but one might make you feel bad after a while.
There are 4 big differences though.
1) Color and quality of the skin tint.
Knockoff usually use 1 single type of TPE and all the details are air-sprayed on the TPE with a stencil sheet. Their skin tones are usually fake with either an orange or a yellow tint in it. That's because the TPE they use isn't white, but slightly yellow and that color is added with the additional color in the TPE. Even worse, they skin color can be off and they correct it with a TPE coating which means they paint it over with a temporary paint that will go off as soon as you'll clean the doll and especially if you oil it.
2) Durability and quality
That slightly yellow TPE is a cheaper TPE as it's actually more alike to industrial TPE than what is used by more "official" TPE doll manufacturers who uses TPE that is closer (if not the same) as medical TPE which is white or slightly transparent gray. The Industrial TPE is more prone to pores since it cure quite a lot faster and its elasticity is less efficient. This is generally why those dolls' TPE feel different to the touch. This means that bacteria and mold can stick of appear quite faster on the industrial TPE. Those large pore (tiny holes) also weaken the TPE against friction and movements when slightly heated. The reason why it's used in the industrial field is because it's cheaper, but also easy to replace. It can also even be recycled in certain ways.
Ironically, the knockoff could be more resistant to heat than the real deal. Some cheap industrial TPE have relatively high heat resistance. Usually between 90 and 110 degrees Celsius. "Official" TPE used is usually within the 80-90 degrees Celsius range. But that's when considering the melting point of the TPE. With the bigger pores, the industrial TPE is far less durable with the movements of the doll.
3) Higher risk of health/skin issues.
Note that this might not be true for all. Someone with tight skin might not have any skin issues... but the industrial TPE is not meant to be in such contact with the human's skin. The oils and materials in it might actually hurt the skin if exposed to it for too long. The TPE used by ripoff might also generate toxic fumes. That's something common with industrial TPE while TPE oriented toward medical uses have to pass some test every month against that kind of chemical effect.
4) You might not get what you paid for... visually.
Here's one thing. If you see a picture of a doll that was taken from another manufacturer, how will the ripoff manufacturer produce the doll? The answer is : They won't. You pay for a certain doll and what you get might not look like the picture at all. You might think you're buying "that doll" with a C-Cup and height while the actual doll could be a D-Cup with a different height and weight. Or, what you see is a doll with a specific type of breasts' forms and slimmer body with a tiny bit of ribs forms at key part and what you get is a generic female body with not as many details that looks "similar", but with far less details in the molding. Then, when asked about the difference, you get the usual reply "The picture on the website was done by professional in a professional set." as to hide the fact that it's a different doll all-together.
