PoP is way more developed and polished than any other mod, not taking mutiplayer ones (just hard to compare as goals are different).
I tried all others, which were mentioned anywhere for comparison or are popular on the forum by amount of replies in threads, but the gap between PoP and any other which would pretend to be second is too deep and too wide.
I really like the idea of those where serious historical reconstruction was made- Brytenwalda and 1257 AD.. And I agree that there is "something" in them. But.... for me they were really uncomparible. As for being hard - I absolutely not agree. Just turn on your imagination and try something unstandard. FOr example - look for larges battle of many lords from both sides , and imprison all or almost all (all non-honorable lords). Go to forests of noldor and fight for noldor as soon as you can - and start taking part in Elven castle tournaments from first month and win noldor staff, or against noldor, getting enormous amount of honor and items. And it's really RICH and SMOOTH mod. You can do many replayes just to try things you could miss. And I never seen any bugs. It coud really be a "Mount and blade 2". It's commercial level, unlike any other mentioned.
And for those who believe it's hard - get to account that it's also the most documented SP mod in community. WIki is amazinig. Mod community is the largest among SP mods. Just do search. And ask questions in threads. It is really, really rewording to get into it.