I have an Evercross H5 and an NIU KQ3i. I like the careful design and self-control of the NIU, but I have had to do a few things. Here’s a reference pic of my NIU:

First of all, on every single screw of the stem, I’ve added Blue Permatex Gel. This has kept them all from loosening.

Secondly, concerning the two silver screws on each opposite side near the bottom of the stem. These loosened often for me, even with B.P.G. I thought about trying Red Permatex, but there’s common advice to avoid this because you’re not going to get the screw out without serious heat, and we’re screwing into aluminum here. So I looked far and wide, and found some thin countersink washers that fit:

from McMaster-Carr (a great source for lots of things, click on the image for detail)

and used one of those on each silver screw. By the time I got that far, I had stripped the head of one of the silver screws, and I replaced it using McMaster:

You’ll notice that my replacement is Torx, not hex; this is deliberate, Torx is more resistant to stripping. Also, that screw is made of a very durable alloy. Have had zero trouble with either silver screw since.

I also tightened the larger hex bolts further down, you can see one of the two in the first pic above.

Lubrication has been helpful also, around bearings and really all over, though you don’t want it on your brake disks. The best so far has probably been CRC Electrical Silicone Lubricant. The “Electrical” part is very important because this is a very electrical device, and we don’t want anything sneaking in and either damaging or shorting anything out!