I have no doubts that once setup in the field there is little to beat speed and ease of operation of a tracked machine.
But that's not everything, a tractor has some advantages, no unloading for one, tractor can tow a trailer of materials and knocker too so only one setup to take to the job- plus if you have a loader tractor you have something to clear fencelines and load fencing stakes.
A track machine is a one trick pony and knocking stakes in may only account for 30% -50% of a jobs time. So you have a huge investment tied up just for that. Having operated both I'd also say that a tractor can be better in some terrains, it's large wheels can adjust better to undulating terrain.
Similarly a digger with the RIGHT knocker, a clearing grab, buckets, auger, wire unrolling capabilities is going to be far more versatile- and you will get a lot of ancillary work ditching, groundworks etc which personally I welcombe as I can sit in a heated can with the radio on on a pouring wet day! I also agree with other- the best digger knockers are homemade by those who know what they want from the machine. The P10 is fairly dreadful- the geometry is all set up wrong for a small digger and it's unstable and makes for untidy fencing.
If however you do mega stretches of stock fencing- plan on towing with the tractor and fence pretty much 100% of the time a tracked knocker may be the best option for you.