Well, I do believe that all safety gear purchased from Decathlon is fully certified and rated for the application. So that's good! & it's not the same as buying sketchy uncertified gear from Amazon, or AliExpress (or whatever).

But sure, there may be differences in the function between Decathlon's own brand Simond and (for e.g.) Petzl, or Black Diamond, or DMM, or Edelrid.

Weight is usually a big one. Some of the top manufacturers of climbing hardware really have put it a tonne of engineering to shave off weight in their designs. A few grams on 1 carabiner can add up over a full rack. Other things are like gate design & gate action. Top of the range carabiners often have a nice gate action, and it's not uncommon for people to have a preferred style of gate (i.e. wiregate, Vs solid gate) to make clipping the rope more fluid.

If you are mostly sport climbing, saving weight might not be a big factor, and in that case maybe there's less to differentiate. However if you're trad climbing, or multi-pitch climbing, or even big walling, saving weight might be a huge consideration.

So all in all, it depends 😋 it can really help to get out with friends and try their gear to see if it makes sense for you. I also watch a fair amount of gear reviews on YouTube, or read gear reviews online.

But yeah, all in all, I do think Decathlon is great for certain items. But I also think it's worth paying extra for those items which you want to function really well (and this can be pretty subjective).