I am unsure about what made it so crash prone
To start with, it wasn't. Windows ME was not much different from 98SE and on its own as stable as its predecessor.
The only plausible thing I could read about it was that it was forked from Windows 95 instead of Windows 98 but how could that be a possibility as it doesn't really make much sense in the end?
It wasn't. ME was based on 98SE with some parts (like the network stack) ported down from Win2k.
What were the technical changes inside it that made it such an unstable operating system?
Now this comes closer to one of the factors why ME was such a failure, as MS did take away much compatibility for DOS without offering any replacement. Like ignoring all installations done via AUTOEXEC.BAT and CONFIG.SYS. Likewise HIMEM handling was changed - and while it improved performance notably, it broke the ability of many DOS programs to run. But not only DOS programs were affected, even rather up-to-date Windows programs could fail due to a changed registry structure.
More important than all technical reasoning, ME was a product and marketing failure
- First ME was hyped as kind of a Win2k 'light'.
- Creating a feeling of buyers being second class.
- Next ME switched (after initially different communication) to be DOS based,
- but at the same time with reduced DOS compatibility
- Even worse, some of these changes crippled a major use for home PCs: Games
- All announced new features targeted only the home market (Not SOHO)
- Creating an image of being just good for non serious work (whereas Win98 was perceived as generally capable).
- MS supported this by pushing professional users toward Windows 2000 Professional
- Indicating once more that ME is not intended for anything but 'simple' home use.
- MS advertisement focused on "easy", "simple" and "just works"
- Again creating the image it's meant for some 'less experienced' user group, not existing Windows users.
- While some internal components were taken from Win2k, no new capabilities were ported.
- Offering no incentive to adapt advanced software to ME
- Real new/improved applications like Internet Exploder, Office, etc. were available for Win98 as well.
- Removing any reason to switch because of applications.
- The desktop was a crude mix-up of Win2k elements with a Win98 colour scheme.
- Thus it was hard to see any improvement at first.
All of this already created a rather negative image upfront, by reviews and word of mouth. Most common a feeling of ME being without any gain. By actual users this was enhanced due to
- incompatibility issues
- especially for DOS,
- and many Games
- a crude UI mix-up between Win2k and Win98 (*1),
- no new functionality (*2),
- additional investment for new device drivers (Device driver signature)
As a result ME combined the spending money and incompatibility issues with the burden of learning a new OS without any benefit of a real new one (for most cases).
A perfect storm and self fulfilling prophecy. Negative reviews create even more negative response with everyone competing to find the bad sides. A product with no need doesn't sell in masses in tech.
P.S.: Archive.Org has got a nice Test of ME from when it was new.
*1 - Colours have already been mentioned, but it goes deeper, like having Network dialogue looking like Win2k, while working like Win98, Menus looking like Win2k but missing functions, or even worse, the items in the control panel being halfway between Win98 and Win2k.
*2 - To be fair, MS did add a lot of features in media handling, game support and power management - just none of them anywhere near a killer feature.