I think this has been discussed in mutiple threads on this Forum, but the "I can't span more than 3 releases" was old school Junos when people did upgrades using a much faster candence. Therefore no one generally never ran into this situation to start with. In the old days, Junos had E-EOL releases, which were Extended EOL releases, and most people went from one E-EOL to the next, within 2 years time.
I do not see the "official policy" from changing, but it real 3 or more generally works fine, especially with certain product families - namely EX and QFX.
My [personnal] view is if you have issues going from any A to some X release, you will run into the same issues somewhere along the line going from A to C to E to eventually X.
All I can tell you is you can go direct from 14.x to 18.x in this situation.
NSSU/ISSU is a completely different situation. Going from one release to another is more complex. Therefore these types of upgrades are support only under specific rules, at least at this time.
In the end, your choice on what you do and how you perfrom the upgrade. Our comments are just other views from real-world experience.