Question: I have various devices to connect to my iMac using good-old RS-232.

Of course, I only have USB connectors on my machine.

I have a couple of Prolific PL2303-based cables, and they seem to work OK, but the kext provided seems a little flaky and I'm not sure about long-term support.

What is the most-stable, best-supported USB-Serial chip-set or cable?

Answer: 
I have two Prolific USB-to-serial adapters but they are made by different Vendors (one is Prolific, the other ATEN).

I've used their supplied drivers just fine. Note that there are open source drivers for these devices too available here: <https://github.com/failberg/osx-pl2303> FYI my use case is actually patching them through to a VirtualBox Windows VM and using them in Windows, so I have two layers of drivers and have not had any problems yet.

However, I think the best supported USB-to-serial devices are those made by Belkin. This is subjective, but I've used them for years without problem, and they are a large company with a reputation. They will likely be around in the future.
Both Prolific PL2303 and FTDI devices work fine.

I'm using a TrendNET TU-S9 v2 currently (PL-2303). Other than the vendor provided driver or driver from Prolific's Taiwan website, options include <https://www.mac-usb-serial.com> (fairly inexpensive driver), and the nice Mac app <https://www.decisivetactics.com/products/serial/> (not free but works well) which has built-in drivers and doesn't require a separate driver installation.

I have also seen claims that FTDI drivers are built into MacOS now but haven't personally tested since I don't have a FTDI device.

One thing to be aware of - to connect to PC's and certain PC-like devices with serial ports (some routers/firewalls etc) you need a null modem adaptor or null modem cable between your USB-serial device and the PC/device you're connecting to, usually with female-female connectors.