It's all about the complexity of the OS. It's, of course, much less stable than iOS or Android, but do remember that these OS'es are designed entirely (from the kernel to the GUI) for mobile purposes.
Maemo, is entirely backwards to that position. It's a full desktop operating system, stripped down, not even for mobile purposes, remember that it has more computer-side functionalities than phone features. So it's more possible to break it up, and there are more possibilities that you can crash it (multitasking without limits has it downside).
So, TL;DR: Yes, it can be an unstable OS. But it exceeds the capabilities of ANY mobile OS to this date.