No, its not 8 nights in a row, you need a 34h rest in between, otherwise it would´t work with the EASA rules. You usually do 6 nights with the 34h break somewhere in between.
It depends on peoples need. the 8/6 is perfect for commuting and you have almost a week to recover, on the flex you sometimes just have a trip of 3-4 days.
The nights themselves can be very fatiguing, when you do 3 sectors every night for 3-4 or even 5 days, when you just do 2 sectors a night, it is usually quite ok.
Depending on how many nights have been worked it is 36 hours / 2 local nights or 60 hours / 2 local nights. Usually this can be done over the weekend when the A/C are in MX.