I have a few third party and/or chinese LEGO-like sets at home, mostly for my teenage son and they range from meh to great.
I got him a Warhammer 40K tank with a few UItramarines from a chinese brand, also the Armored Personal Carrier from Aliens for his birthday because he really loved that vehicle in the movie. And a few Gundam-like robots and a Pacific Rim Jaeger too I think some years ago. I got a Titanfall 2 BT for myself some years back and it looks really cool but it doesn't really hold up as far as stability and solidity goes, you can tell it's pretty cheap plastic (though official LEGO also break after a few years, especially the clips on bricks tend to get really brittle).
I also got him some years ago some cheap train sets because the real ones he saw on Youtube were long out of print and/or at prohibitive prices, those were pretty sturdy.
I also got myself last year a 3D map of Hyrule from Link to the Past, around 12" x 12" and all made from bricks, it looks pretty damn cool and was pretty easy to build, but I remember I had some problems in the instructions because they're not as well designed as LEGO stuff and sometimes the colors aren't exactly matching on the instructions and in the actual bricks so you have to figure it out. Again, not a lot of experience for me on completely original designs.
As far as instructions go, some chinese companies have booklets so I recommend keeping those because you may not easily find instructions pdf online if you need to rebuild it, but most these days have a simple piece of cardboard in the package with a QR code you can to get the instructions on your tablet.
There's also some pretty decent third parties that aren't knockoffs but just other brands, and fully compatible with LEGO stuff since that patent became free a few years back. My son likes some military brand (Sluban) that does tanks, planes, rail cannons, aircraft carriers, etc.. it's pretty cool.