Yes, 75 USD is a high price for a virtual aircraft (costing more than the simulator itself), so that puts a certain demand on the quality of the product. I can confirm that as my only XP aircraft, the IXEG 737 creates a jerky movement in the picture which is very noticeable when taxiing and looking out of the captain's window and also outside view of the aircraft. The stutter occurs every 750 ms or so and is not related to overall fps.
As for the VNAV functionality, this is of course disappointing that it doesn't work properly given the price tag. So the FMC/FMS needs the biggest overhaul, but reading posts on this forum I know that IXEG are well aware of this. With a lack of proper ATC, VNAV is important, or shall I say convenient, when flying STARs. The OP mentions the 737 Zibo mod, but even in the latest version, I found that VNAV didn't work properly there either. And even PMDG's VNAV fails, albeit rarely.
The IXEG 737 looks fabulous, and the flight model is very good. The aerodynamic feel and feedback from the plane is probably unrivalled on the market, especially during landings. All the 737s are quite nimble in handling in real life and I think the IXEG 737 exhibits this as well.
I think something in X-Plane v 11.20 didn't go well with the IXEG 737. I've tested the aircraft twice, and both times, it had thrust issues with different power readouts and performance (on my first attempt, the plane nearly swapped ends on the runway due to the difference in thrust between the engines) and difference in N1/N2 performance and fuel consumption. See the attached image for the readouts, this at cruise level.