Hello Nevtral! It's a bit complicated with The Ordinary: they have so many different products, many of them are great, others are less effective. It also very much depends on what you are trying to achieve. A downside with mostly relying on The Ordinary for your skincare is that people often end up with a lot of products they are supposed to use daily (while some actives can be happily combined in a single product from other brands) - so than the price advantage of the Ordinary disappears. The other downside is that many products contain too high of a concentration of actives (e.g. their Niacinamide serum - 10% is bit high, most studies with this active were done at 6% and it's enough. Or their infamous 30% AHA+ BHA peel that is way-way too strong to use for most people unless they are real skincare-pros and know their skin very well.
Also, some of their products use a form of actives that is not particularly bioavailable (some of their Vitamin C serums), so that's often is a waste of money.
But on a huge positive side is The Ordinary offering in the retinoid area: their retinoids use highly effective actives, in good concentration, and the price is unbeatable. Plus they are available widely.
Their azelaic acid serum is also often the only available affordable formulation in some countries.
So the bottom line is: yes, many of The Ordinary products are very effective, but some aren't. It also makes sense to watch their ingredient lists closely if you have sensitive skin: sometimes common irritants like Rosmarin or Witch Hazel extracts sneak in (ok for most people, but risky for reactive skin).