I actually don't understand the comparison with Fognini but I am not surprised Nick hasn't cracked the slam SF duck yet. SFs are not that straight forward to make, you have to have the right form and likely a bit of draw fortune. However not making QFs shows there's work to be done.
The biggest Kyrgios issue is just the fluctuation. I recall that Thompson match at Wimbledon. Kyrgios looked bloody awful at times. Thompson lost ultimately due to passivity at the end but very nearly won the match. Nick was very fortunate to escape that one. The previous year, he also got decked hard in straight sets by a returning Nishikori, his serve broken 3 times by the start of the 2nd. Kei finished the match before the sun went down and showed little sweat despite grass not being his best surface by stats (though small sample size and injuries contribute).
The other big telling match was the loss to Andreas Seppi at AO in 2016. Seppi got his revenge for the 2015 loss but the way it went, Nick suddenly lost steam from two sets up! He played ultra passive, even his serving slowed, and couldn't finish pts but tried to outlast Seppi (didn't work). Even on MPs we was just listless and hoped Seppi would make a mistake. Seppi just remained composed. It was one of the most apathetic displays I've seen from Kyrgios, not even reacting in general.
The BO5 scenario makes it more serious too. You must carry momentum and application across potentially 5 tough sets. On some occasions we see that (the comeback from behind win over Gasquet in 2014 saving 9 MPs come to mind), other times we see a disinterested or undulating Kyrgios who shoots himself in the foot with poor application or consistency during a match. His results at the USO are particularly disappointing.
Of course mentality and work ethic aside, there are aspects of his game that need work here and there - movement, return and BH. Areas that aren't exactly awful but they can be exposed badly when Nick's mind isn't on the job.
They are both called genius (but I disagree with McEnroe's comments about Krygios),
both called flashy shot-makers (especially Fognini, but Krygios sometimes),
both tankers, both erratic personalities, and that is why I said Kyrgios is a serve-bot version of Fognini.
Fognini matured in recent years, but still hasn't made a QF since 2011 Roland Garros, so he's still an unfulfilled potential and so is Kyrgios.