I have had exactly the opposite experience as yours. You describe every flat pack furniture brand except Ikea.
The only thing Ikea doesn't do superbly (for the price) is padded furniture. Their chairs and couches are terrible, in my opinion. Their cabinets, though? Their desks? Great look, and reliable for years. I am proud of my Billy bookshelf and the books on it in the living room.
I think you're both a little bit correct. Ikea probably sells the best quality flat-pack furniture out there, but I think it's pretty fair to expect their stuff to fail in some way within 5 years of normal use. Keep in mind the failure may or may not be catastrophic, and the item may still be usable for some time. They're definitely a good value, but only if you're OK with eventually needing to buy replacements.
Some personal anecdotes:
Lack coffee tables: utter trash, fell apart quickly (in a way reminiscent of what the grandparent post described)
Galant desk: still going strong after 12 years and 3 moves.
Aneboda dresser(s): Got three, 100% track failure after a few years and no longer suitable for daily use.
Billy bookcase: re-shelving books pushed the backing loose, and the thing looked like the Leaning Tower of Pisa until emergency repairs were undertaken. Will be junked in the next move.
Six years ago I bought one of those longer Lacks for ~15EUR and heavily use it close to every day. The black top varnish seems to get pretty thin at the most used locations which is the only wear marks I can spot.
I use it for eating, writing, etc. Maybe others have other plans to use a coffee table in a way that requires tougher structures.
I think lateral stresses (from people's feet, etc) did mine in. The seams failed, and things quickly deteriorated after that. A lot of Ikea stuff is designed with zero redundancy or design margin, so if any part fails or weakens the whole structure is compromised.
From what I saw, they're made out of a tiny amount of laminated cardboard. I personally wouldn't trust it, but maybe it would work if all lateral stress was avoided.
I consider the Lack tables to have a functional service life of two years - if its lightly used, much longer - but under daily use, just buy a new one every two years - even with that - after 10 years, money wise I'm still ahead of more durable furniture.
Are there other flat pack furniture brands? I did not know this. I had all IKEA up until I started replacing IKEA with already-built pieces from garage sales/ furniture stores/ family.
Maybe I have just gotten unlucky, and fallen in the tail end of their defects.
Sauder is the only other one I know of. Their stuff seems to be pretty well designed and of good quality, though I wish they were more unapologetic about being fiberboard, as I'm not a huge fan of fake wood textures.
As a counter point, ikea furniture works well enough for most loads but I have a chest of drawers and the bottom of the drawers is made from something similar to a less stiff version of masonite. for clothes it would be fine but for anything else the bottom of the drawers bends and scrapes the next drawer when opening. I need to retrofit the bottom to something worthwhile.
I've also never encountered those issues mentioned with Ikea furniture and have had them with others. I'm not sure if they were technically "flat-pack," but many years prior to seeing or knowing what an Ikea is, I've assembled things like computer desks and TV stands. They all had the issues described: lament peels off, too flimsy.
There are (from searching) several supplies of cheap, flatpack furniture for landlords, which probably explains why half the properties I rented as a student had the same furniture, made of 5mm thick hardboard and bits of plastic. That stuff didn't even hold together properly when it was new, let alone a couple of years old.
Habitat should be decent quality. MFI used to be a major retailer of flat-pack stuff, but they went bankrupt in 2015. My parents bought almost everything for the kids' rooms from MFI. None of it is broken, 15-25 years later.
That wardrobe's not really a flatpack, though. Last time I looked, Argos's flatpacks were similarly priced to IKEA's, though I haven't really looked in a while.
The only thing Ikea doesn't do superbly (for the price) is padded furniture. Their chairs and couches are terrible, in my opinion. Their cabinets, though? Their desks? Great look, and reliable for years. I am proud of my Billy bookshelf and the books on it in the living room.