The BPM really doesn't necessarily change that much in modern music, zentara. Sure, there are songs that speed up and slow down, but these days most music is recorded to a click track or uses programmed drums. And even though music such as jazz is highly syncopated and the rhythms are highly complex, the drummer tends to keep a very steady beat with the hi hat. There are, of course, exceptions to the rule, so to speak -- but i think that you can get an accurate BPM measurement programatically with most songs. For those songs that do change time signature and tempos, you'd have to break them down into parts. For those that feature a drummer that can't keep good time, you'd probably have to take an average and round that number up or down to the nearest commonly used BPM.

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

Quick, what's the BPM of Steve Reich's "Drumming"? And György Ligeti's "Poème symphonique"?

The BPM really doesn't necessarily change that much in modern music

Thus speaks someone with very limited musical tastes :-)