Well, I would say Android is actually better of compatibility wise, at least with the Java-like programs that don't have to worry about binary compatibility.

On the other hand on Maemo, I don't think binary compatibility was even considered and even when running Python programs you have to handle stuff like:

• different location info access (gpsd in Diablo (?), liblocation in Fremantle and QtMobility in Harmattan + Fremantle)
• different UX (minimize + close buttons on Fremantle vs Swipe on Harmattan)
• different GUI toolkit availability (GTK+old Hildon on Diablo, new Hildon & Qt on Fremantle, QML on Harmattan, no official GTK/Hildon)
• icon sizes (64pix for Fremantle, 80pix for Harmattan)
• distribution channels (Extras for Diablo & Fremantle with dependency handling, Ovi store & AFM for Harmattan without depndency handling (so far) & OBS for package building)
• keyboard (no keyboard on the N9)
• different sensors (no magnetometer on then N900, only N9 has NFC)
• security (Aegis on Harmattan complicates packaging & testing during development quite a lot)
• update/software combinations (vanilla Fremantle, vanilla Fremantle with Extras-devel, Fremantle CSSU with Extras-Devel, fremantle CSSU without Extras-devel)

On the other hand, this motivates developers to write more robust & platform independent applications that handle various edge-cases gracefully. You also learn quite a lot about how the platform actually works inside during development.

Does Mer (the original incarnation) count?

Yes, you can. A 770 can run the first 3 in official and the next two in "Hacker Editition" releases. An N800 can officially run Bora, Chinook or Diablo as well as Mer (original and possibly current), likewise for N810* modulo Bora.