Done a few - and they were fine. Quality varies, but the main issue as with any kitchen, flatpak or not - measure carefully,especially noting whether walls are plumb or not and which way the slope if they aren't (and most aren't!). You can always do something to fill a gap - damn hard to shorten a unit that's 5mm too big for the space you have - or even 1mm! levelling too is important - and depending how the floor is has to be done progressively once all the floor units are in place. I know many don't like the idea, but I do the wall cupboards first as I find it easier to work straight at them rather than leaning over the floor units.
Think hard about all doors and drawers opening dishwashers opening, fridge & oven etc. If you are not the main cook then get them to look at it too - and remember the cook's triangle - if you can have no more than 1.5m to 2m between each the three main utilities - fridge, stove top and sink. bench height is important - go higher than you think unless you have small stature people,likely to use it - IMO minimum 900mm - I have done 950 and even 1m that work well - real backsavers. Another thing that is not done now is a wide overhang. Benchtops tend to be flush or have lip of maybe 20mm or so - I use 50mm and the reason is that spills go onto the floor and not not cupboard fronts and it gives a better and bigger work area for little cost.
Summary - measure carefully and really spend time on layout and what goes where!![]()