In general, their brick quality isn't on par with Lego. The clutching of Lego bricks is finely weighed and consistent. The colouring is brighter. They don't have nasty toxic smells and paints. The set catalogues are more diverse with many more themes. Set designs are original and matching the age group. There is also historical significance and nostalgia for certain sets and themes, and they can be re-released as new sets with modern building techniques and new parts.
Is this based on your own recent experience? I recently got into the mood to build some LEGO again, after not having done it since childhood, but I recoiled at the prices of current LEGO sets. Got something from BlueBrixx instead and am really happy with it. The bricks look great, feel satisfying to put together and I can't tell a difference from how I remember it from all those years ago.
I definitely remembers seeing some Mega Bloks some 15-20 years ago and they were really really bad like you described but from what I can tell, other competitors have actually caught up to LEGO nowadays.
They really aren't that far off these days. Certainly no 'nasty toxic smells/paints'. Brick fit/clutching can be a bit sub-par on certain parts, but is generally not far from the real thing.
Lego does a much better job of instruction books than the 'fake Lego' sets I've seen, though. Also, Lego has higher standards for the model designs themselves, whereas some of the 'fake Lego' model designs can be rather more fragile than anything Lego would release, which adds to the impression of poor brick fit/clutching (e.g. the Xingbao Xenomorph)
(Haven't seen any of the Technic clones, though. They're likely to suffer more from loose or poorly-fitting parts)
