I remember a W1GHZ project from the 00s or maybe even the 90s where he eventually got hairpin filters for low microwave frequencies to work using cheap PCBs.
Generic fiberglass is NOT consistent across multiple mfgrs enough that you can spec it for mass production from any mfgr in any country at any time, you need the very low variation specified "RF PCB material" to mass produce precision untuned hairpin microwave filters.
HOWEVER, the cool part of the project was generic fiberglass is consistent enough across one mfgr over a short period of time, that you can characterize product X shipped from PCB house Y and then you can very consistently get boards at the proper resonant frequencies.
The meme was not overly successful in the amateur radio microwave community, sadly, but the project did work perfectly well at the time.
Even the 4 layer board processes are not set up for good power integrity. They have a center core for layers 2 and 3, with 1 and 4 as foils. It ought to be two cores with prepreg between, giving the low impedance for internal power planes.