Still worth to buy a Mining Rig?

I would say no. I mined Bitcoin and alternate coins for quite a while a few years back, and even then it was really hard to make a fair amount of money with it.
However, if you can find the right coin and play your cards correctly you may be able to turn a profit. You essentially need to find a relatively new(er) coin or one that has a lower difficulty level that you believe to raise in value. Once the difficulty level has increased quite a bit, hold onto the coin and move onto another.
After you've developed a pretty good collection of several different coins, (and they have gotten to a suitable value) sell them. This is what I did a few years back and it worked out quite well.
There are several websites you can use to help you out with this, such as CoinWarz and Coinmarketcap. Furthermore, there are also mining pools that will automatically have your hardware go for the most profitable coin at that point in time, such as Multipool.

Cryptocurrency is a lot like the stock market. You need to have money to make money, and you're not going to strike it rich overnight unless you get UNBELIEVABLY lucky.