A cacodemon ate my original post, so I am tasked with the labor of recreating it as far as I remember.
IMO, you could probably always make any one of the Ripper, Cerberus Armor, or Cerberus Boots with little difficulty on nearly any run due to them not requiring special mods. You might have to scrounge a bit and save up most of your mods, but they're definitely the easiest Masters to build.
As for the BFingG or Cybernano, because these require two special mods each (fffffff two
Firestorm, you will be mine one day, BFingG), I find the odds of building either of them outside of a specialized Tech run where you use Scavenger to scrap everything you find with the goal of making them, to be rather slim to none.
So, that leaves us with nanomac ammo and demo ammo. Call me biased, but I would assume most people upon receiving a nano mod pack are way more likely to assemble Antigrav Boots (not Master, practical and easy to make) first before nanomac ammo, and demo ammo is rather situational. I
like them both, but as Napsterbater pointed out, you don't
need heavy firepower, and you can get your hands on a BFG without too much agony, and a nuclear plasma rifle (even though the wiki says it's random, I always get it on the Spider's Nest. Luck? Or is it confirmed Spider's Nest reward?) pretty much all the time.
My personal preference is to use envirosuits (since I usually seem to end up with quite a few) rather than use up a ton of mods on Cerberus Gear (which is dangerously slow for my playstyle, and doesn't really offer me much advantage over envirosuits other than the knockback or plas reduction), and I shy from melee outside of specific challenges or theme runs, so I have no need for a Ripper. So for me it boils down into three undesirable Masters, two (nearly) unobtainable Masters, and two "ehh if I get an extra nano after building anti-grav boots or if I find a ridiculously sweet special worth modding" Masters.
So after a rather long winded explanation,
How frequently do you find yourself making Master assemblies?
Rarely, if ever.
are they worth it?
If you have the mods for the BFingG or Cybernano armor? Sure. For the most part? No. You'll probably get far, far better use out of your mods by either making lower level assemblies, or just kitting out a weapon with 5 mods to make a death arsenal.