Timing will make a big difference. Beware of new start ups and the much hyped "prelaunch" promotions.
Roughly 98%, including network marketing companies, are gone within 4 years.
Some of the prelaunch programs you see will never be a program. They have no intention of actually having a company. They will use special incentives to get tons of people to sign up and then promote the site to get others to sign up.
Just before the launch date, they simply disappear. Now they have a very large list of all the people who have signed up and they can sell these as “targeted leads".
This is one of the reasons why buying leads is always a rip off, but that is a tip for another day.
On the other hand, a company that has been around 20 or more years runs the risk of flat lining, meaning that they experience very little growth.
The biggest growth in a network marketing company is usually between years 4 and 10, but growth can remain fairly good for several more years depending on the company structure and products available.
Do your homework on any company you are thinking of joining.