Here are a few examples for hardware:
I bought an AMD CPU with GPU cores on it, about 3 years ago. Immediately, I was able to verify that the CPU was usable if the BIOS settings were set properly. The whole time I had to use a separate graphics card for smooth video, or use the VESA driver. It was only until a few days ago that I was able to use the graphics capabilities from that GPU, (without VESA) from their improvements to the drivers on FreeBSD 11-Stable.
I bought a Bluetooth game controller, which could also be used through a USB cable. I was unaware that Bluetooth versions past 2.x were unsupported, and versions 2.0 were poorly supported or unsupported at all, depending on the driver. Its capability through the USB cable, without Bluetooth, worked, but it took a lot of reading conflicting and inadequate documentation trying to test configuring it, and the way to do it is still not standard. Any more recent Bluetooth version capability may take a while, and many don't expect it at all.
Another one is MIDI instruments that don't use USB. I never tried it, but if it is to work, it will work with the sndiod MIDI server and other utilities from
audio/sndio. Still, the API to many applications to that server is not ready yet. I also haven't heard of anyone getting it to work with MIDI instruments from FreeeBSD.
In the past, I've had trouble getting a scanning, printing and all-in-one HP device to work. It has worked in the past, and several other times, when I tried to make it work. It takes a lot of interpreting documentation, and sometimes trial-and-error. Removing an option from the Kernel can also prevent functionalities of it from working. Setting up the right X11 fonts is also important, depending on who you're printing for.
Working on something, and putting together notes for others to read, perhaps helps others test out bugs and try out drivers.
I tried to avoid GCC and binutils in the Base system:
GCC could be avoided fairly easily, but not always for ports.
Through a lot of trial and error, by avoiding binutils, I've made my system not upgradable several times. After trying, I still don't have the knowledge to get back to an upgradable system, without fully reinstalling. Right now, I'll settle for binutils.