The only time the neutron jumps won't work is if you need to go way off a direct route between your start location and target location, in which case as you say you'll need to use an external route plotter such as spansh because the game has to apply some limits when calculating the potential 'next jump' in a route. Every jump is calculated along a route which is basically a cone extending from the last one and if you'd need to go say 100LY straight up to stay on a neutron path the plotter will just crap the bed and nope out. For a direct route though, setting the galmap filters appropriately and including the 'jet cone boost' option will plot a muliple jump route using neutron boosts (or white dwarf boosts) when they're available just fine. You might have to unselect and then reselect neutron stars and white dwarfs in the galmap filter after you've selected your target system to force it to re-plot correctly as it can be a bit quirky occasionally but other than that it's fine.
Like I said in my earlier post I assumed Starfire was talking about jumponium boosts. To plot those you need to enable the entirely separate 'FSD Boost' option on the galmap but even with it set, if you try to plot say a 250LY route thinking it will give you two jumps of 100LY and a third one of 50LY it won't - it will only ever plot the first 100LY jump when you apply jumponium and after you make that jump, you'll then need to plot another one.
Seems that was what Starfire was talking about based on their more recent post. And yes, it's irritating.
