I'm using the Diplomacy mod since long and never had crash problems related to it (as fas as I can tell from the crash reports). I have generally a very stable BL (with about my normal load of 20 mods) and crashes very seldomly.
I would advocate to try Diplomacy. For the messenger system alone the mod deserves to be used. The diplomacy itself is maybe a bit better than vanilla and offers more options, but don't expect wonders.
I'm currently playing with RBM and Banner Kings and Diplomacy (and the usual mods, like Improved Garrisons, Detailed Character Creation, Arena Overhaul, RTS, Break In, Kill Them All, Configurable Battle Scenes, OSA, True Sieges etc.), so far no big problems.
However peace is a relatively constant feature in the playthrough and Banner Kings can make peace quite annoying. Not only does it make troops much more expensive and troop upgrading slower (therefore I use a mod to train troops), it also makes it very difficult to get your first fief. It may be nice to use Banner Kings options if you got one, but in the one war I had since start (as Khuzait) I was not even on the vote list for a fief. So in 1088 I'm still roaming around as fiefless vassal, one of the most boring experiences in BL for my taste, I have an expensive and slow big party without the possibility to park troops in a fief and do some other stuff; disbanding would be a terrible option because of the very slow leveling in Banner Kings. A bad concoction ...
Take also into account, as Banner Kings has a kind of marshal system, that as vassal you are mostly hindered to play actively for a longer time. In vanilla you can summon a small army to help others or defend your fief, in Banner Kings you cannot till you have high rang, you have to trust the AI to help you if you have a fief under attack. That may be realistic (and fantastic for the Warband fans), but it is a questionable realism, like week-long recovery for heroes shot in a FPS. We all know that the AI in BL is "brilliant".... I will delete Banner Kings for my next playthrough, it's features are not a compensation for the vexation it offers. Of course that's personal taste.