After going through them, it was looking like if I wanted an OpenWRT device for ≤$50 US, I'd be buying either an Archer C7 or a GL-AR300M. Amazon sells refurbished C7s for $50 and new GL-AR300Ms for $40, both with free shipping. The average price of used C7s (including shipping) on eBay was $35 in the last few days.
I very much liked the GL-AR300M as it comes with OpenWRT by default. In fact, its page on Amazon is basically a panegyric to OpenWRT, touting its beautiful strengths and universe of features. Its clear this router was designed for people like me. It comes with dual flash areas (128MB NAND, 16MB NOR), but you can get it for half the price if you opt for the "lite" model with only 16MB flash.
However, I don't need a pocket sized router and rewiring the installation site to provide 5VDC power over a MicroUSB cable would be inconvenient. The GL-AR300M's lack of 5 GHz would be a serious problem in the congested urban area where I'm planning to put it.
End result: I'll likely be getting a used Archer C7 v2 for this installation. However, at a different location, it looks like I'll need a much beefier device, so I'll be re-reading this thread as I compare the more expensive routers.
Edit: Added GL-AR750 to table. If the 128MB NAND isn't usable on the AR300M, the slightly more expensive AR750 seems promising: it has a MicroSD slot for extra storage. Other nice features: 5GHz band, WiFi AC, and (optionally) power over Ethernet.
NAND support for ath79, in general, is waiting for Kernel 4.19 and the upstream spi-nand framework. GL.iNet supplies a relatively current, "close to the tree" OpenWrt and package epositories, as well as complete and functional source tree on the ath71xx target that does support NAND.
If price is not an issue, and you are looking for one of the best packaged devices that runs OpenWRT, I would recommend the Netgear R7800. I work as an IT consultant and have installed dozens of OpenWRT devices at client locations. Over the years I have used several different models as my standard unit, including the TP-Link wr1043nd, wdr4300, archer-c7, and c2600. But the R7800 out performs them all on range and throughput. The only downside is that it costs about $200. The TP-Link c2600 was a pretty good deal, and came reasonably close to the R7800 in performance for half the price, but it has been discontinued.
The APU2 isn't a turnkey solution, especially if you need dual band, simultaneous wireless. Not that you couldn't engineer something based upon the APU2 that would do that. But for a simple wireless router, the R7800 is a nice package.
If you don't need wireless and are just looking for a good, open source, high end router, I often recommend pfSense (usually running on hardware I have built, but sometimes running on an off-the-shelf appliance).
Simply depends upon interpretation of "Top ten routers". In case, it means "Top ten most powerful routers" APU2 is among the leading ones.
Also, regarding reliability.
But you are correct, APU2 is not for "Plug and Play" usage.