Well, good question, indeed.
I deal with computers since only 33 years. And windows was not the first system.
What I could experience updating incrementally works fine in most cases. But have a full update (relating Android) is nearly a full system change.
Even doing so on Windows bear the risk of lossing data.
But there you can generate a full backup e.g. with Acronis and a simple boot recovery.
Not so easy with Android as I could read. It surely would need more steps than on any conventional computer I believe.
I keep the most of data on my sd card if possible. For updating of course I would remove it (also for a backup) and when the Phone update if done insert it to see how things are working.
Individual settings may be lost, well…
Many users asked for a Marshmallow update again believing in miracles to have every issue with Lollipop solved…wrong expectations again. This is not the way operating systems work. It is not a simple (incremental) update, but nearly a system change. Yes the expression “update” does not always has the same meaning.
Each update should solve issues, at least clear security issues. But as I wrote in another post in this forum already, new issues may rise. That is what we have now.
If there would not be anyone to update to Marshmallow FF may be somehow disappointed and pi…after all these requests. What - users wanting a major update but now actually not updating (for free).
Anyway there are always curious users doing the first step. I am not so eager with this step.
But sometimes I was among the first in case of incremental updates for Lollipop in the past and each of them got things on my device better if there were any noticable issues.
My Lollipop performs flawless without any annoyance for my needs. But I do read and have expected many issues coming with the first Marshmallow update version.
The good thing is, it can only get better, but there is a need for new updates/fixes. At least FF has its monthly schedule for us to await updates, not several months as I know from other phone manufacturers.
There is also a difference if Marshmallow is received by an update or a fresh installation (from the manufacturer) due to individual settings (rooting) and propably there will stay some remains from Lollipop left.
This should not be much different than updating any Windows or do a fresh installation.
There are users who has updated facing no troubles at all (maybe the majority with non-rooted phones as I have??). But who can tell for sure in advance.
At last it is up to each individual whether to update or not. Maybe I will try soon should I have some spare time for dealing with my troubleless operating phone some more.
Anyway it is highly to appreciate that FF took all these expenses to bring this update. This is a further step assuring longevity and winning more customers.