The arm extender is not compatible with any weapon that uses Bettter Combat mechanics

As far as I know, Better Combat deliberately isn't compatible with Reach Entity Attributes, which is what the Arm Extender (and some other mods) use to modify reach.

Plus, there's this bit from the Better Combat FAQ.

As explained by BasiqueEvangelist above, this is not something we will fix - it's intended behavior by Better Combat after all. If you still like this to work, you may want to open an issue on their side and maybe ask about a config option

Cheers