The Golden +0dB

Really, how much problem is there with achieving a “perfect” +0dB signal chain throughout?
Why should there be any problem? Does the problem of gain variations arise from miscombining certain components and such?

Is there a technical challenge with all analogue interconnect terminals, which aren’t even optimized for least-mass and most precise mechanical connectivity? Are there specially designed setups with optimized/no analogue terminals at all, meaning here, just welded well?
I’m debating here, how much do standard connections make the +0dB wobble, like, as in +, -, how much?