Cheap imitation building blocks are stunningly high quality nowadays and many companies offer extremely interesting sets at a fraction of the cost of real LEGO blocks.
I used to be very pleased with the Polish "Cobi" range. They make a lot of military models, which were great for my dad, who has mobility issues that made traditional model-building difficult.
But then we bought one of their biggest sets-- a Maus tank-- and he complained it was missing parts and unmanageable. I suspect thousands of gray-on-gray parts might have exceeded some unknown challenge threshold.