I finally gathered guts to ask this question here. It might sound basic and stupid. But search could find me an answer.
what is generic build image? can it be installed on any router with specific Chipset?
what is vmlinux.bin generated what is it used for ?
What is your goal and perspective?
I had been building openwrt for my routers. and using specific images generated . Never bothered what other images are for. So I thought I should try to understand what images generated mean and where can be used.
like if i do generic image it will work with specific wireless chipsets. And all the routers which are using. ( because gpios are configurable but , basic like wifi antenna ethernet port pins and ram and rom pins on the chips dont change.) So LEDs might not work but the basic router should work.
Somewhere a member here said that he used a generic image and it worked.
importantly, they said that in the context of a specific board and release and likely clarified what "image" meant ... your question is a good one... and I also want to know more on the topic 
do you mean sysupgrade.bin when you say "image"?
i think some confusion on this might stem from how things were done in the "pre-dts" days... which predates my involvement... ( i could be wrong ).... as we still dont have context about that... it's hard to give you any specific answer....
the more specific you are, then the more likely it is that you will get specific answers...
on ipx806x master/snapshot for instance (practically)all *.bin files have generic in the name.... can you see how non-specificness might lead to a fast-brick 
I think that generic thing is cleared to me now. its when the guy said generic probably its not counter specific.