Most network marketing revolves around convincing people into believing they own their "own business", when in reality they are a salesman working on pure commission with no set schedule or benefits. Your income is based on whatever amount of products you can sell - usually with a strong emphasis on trying to collect money from your friends and family (using your personal connections to sell product to people close to you sounds a lot less sexy than "taking part in an exciting network marketing opportunity").
The more nefarious network marketing companies require you to purchase the goods you will sell in advance of when you sell them. You buy 100 (or 1000 or 5000+, often "discounts" on bulk purchases which will "increase your profits") worth of goods in advance (which they call an "investment", which is not what people mean when they use the term normally), and then you are stuck trying to sell the product to people you find do not want it.
Not having to buy product in advance, pay for training, or make an "initial investment" is not really a guarantee of anything.
The thing is, when you cut through the copious loads of horse-manure networking marketing companies wallow in, you have a simple underlying mechanism: a company has products to sell, and they want people who will be a salesman for them but for whom they have to invest little or no time, money, or risk to train. I.e. - they want that salesman to be you. Since they are not employing you directly, and all your income is based on your ability to sell the product, they also know - with a wink and a nod - that you are strongly incentivized to say and do things that they would be held liable for if you were their employee, like exaggerate or lie about how great the products are. A lot of "nutritional products" get sold as disease cures, panaceas, miracle drugs, etc. - things the company could be sued for if they said it, but since you would be an "independent business owner" the company pretends they have nothing to do with it.
As to interest, compound or simple, most network marketing has no such concept because it isn't about interest, it is about commissions on selling products/services.
As to whether or not any of this is good, it depends on whether you understand the core mechanism of how things work, what the product is and its quality/reliability, and if you want to be doing this work as a salesman. Is the product legal, of good quality, and does it serve a customer need that is not made-up or grossly oversold? If all this is true, why do they have to go to such lengths to sell nonsense about owning your own business and being engaged in "network marketing" instead of just telling the truth plainly, that they want you to be a salesman for their company but without being officially employed or having a guaranteed income?