The main thing it boil's down to is STABILITY! Like someone else already stated, the downtime is your biggest loss.
If your rig is not stable its mean you overclocked / undervolted your cards way to much , or you have a bad bios mod .Members dowloading bios from the internet and flashing those bioses to they cards ... They not even sure what is they bios version number , flashing your card with different bios version create instability on your system !!! and can brick you cards ! or you going to get hashrate jumping up and down !
Down time ? If your gpu not hashing , its mean that gpu not working , check the cooling fan if its stopped , thats the bad card , turn of you 21 cards rig , remove your non working cards , and you ready to go with 20 cards ... down time is a minute or less ... Put that card into your computer and see what you can do with it ...
Or if one of your card have a problem like not stable hashrate , turn off that card with your miner program , wait 5 minutes , then that card going to cool down , so the cooling fan going to stop , so you know what card you need to be remmove , during this time you still mining with your 20 another cards ... turn off your mining rig , remove that card ( 1 minutes down time ) , then start your rig with 20 cards , and fine tune your removed card with OverDrivenTool or any tune program in your pc ! check for memory errors with HWinfo ...
That's I mean, I agree with you. Didn't all miner use one type, one vendor in one rig. One of my rigs has 1xRx 480, 1xRx 580, 1xR9 295x2, 1xR9 290, 1xR9 290x, the whole cards with a different vendor. The other rigs have same type and vendor. So we can't generally say that how easy to build 1 rig.
That is why You always have to fine tune each cards separately 
There is a option in afterburner for that , or you can use OverDrivenTool to set up frequency / voltage for each cards