Linux

I interviewed/hired C graphics programmers off and on for 20 years. 95% of candidates had near 0 actual ability to draw a sine wave on the screen, given example code that draws a rectangular box to draw the sine wave in. We pre-screened the applications for appropriate experience, so 100% of interviewed candidates had appropriate experience or academic background claimed. About 2/3 of the candidates "talked a good game" but it was literally less than 1/20 who could actually make lines appear according to a math function WHICH WAS THE CORE OF THE JOB. I tried giving clues. One intern level hire I gave 3 heavy hints to, basically doing the test for him. He never did learn to do much of anything for himself even after a 4 month trial period. Then there were the ones who got it, and they performed the test like a hot knife through butter. One candidate took the (time series simple sine wave) test before we paid him to travel for an in-person interview, and in person we sprung a "now, do a polar plot of sin(t) on X vs sin(3t) on Y" - he aced that too, we made an offer - then he discussed moving with his wife who he assumed would be fine with it, oops.

AI agents may not be great, but in my experience they beat the hell out of the advertise, interview, hire process.

Ex-hiring manager here.

We preferred certs that were tied to specific software solutions.

So the A+, Linux+, Sec+, LPIC, etc. were fine but those are generalized.

We looked for Red Hat certs, vmware, aws, etc. because that was the software we used.

Of course general Linux skills are sought after, but less training was required for the specific certs to a certain sense.