Ikea oscillates in quality. Originally it was super good, then they had a cardboard period (1990's) and then when they started to be associated with 'painted cardboard' they upped their game and started to make quality stuff again (easily distinguished from the painted cardboard by the price). The last couple of years it has indeed been downhill and very rapidly so. I've found that a sharp tap on a part will tell you all you need to know, the cardboard will give and flex whereas the solid parts will hold up nicely.

The honeycombed structures are quite strong for their weight but not strong in absolute terms and you definitely don't want a book case made of that stuff.