To the original question- overbuilt?
I don't personally believe so with the.exception of the dual Z motors which appear to merely be a carry-over from previous smaller printer designs.
They often have dual Z motors for various mechanical reasons.
If anything the R1 FT-5 is a little under-built in several aspects for its size and speed.
I cannot speak for the R2 version as I have no personal experience with one.
I would again use the stork standing on a spindly leg in an earthquake analogy.
There is indeed a lot of rapidly accelerating mass high up and relatively little cross bracing to stiffen the vertical framework.
Both I and others have added triangulation straps or plates to greatly stiffen it.
And for mine it did make a sizable improvement in print quality and reduction of overall vibration.
Others have reported similar improvements.
On a different but related note (I know this thread is mainly about motors and framing), the electronics enclosure fan and vents are woefully inadequate and many people including me have installed far larger ones in the back hatch.
The original PSU was barely adequate wattage-wise to start with, and its 12 volt operation was both inadequate for the stepper drives and borderline dangerous due to the high currents necessary for the bed heater.
The original controller board without an external current booster was a potential fire hazard at 12 volts for the same reason.
The original direct drive extruder was marginal (at best) in my opinion, and the cooling fan for it was as well.
If you put any smaller motor on the original extruder it is unlikely to work properly (I tried it and it failed miserably).
And there is so much more.
All that said, as with anything else in life there will doubtless be others who are able to produce ultra fine details on a 14" high print at high speed with good layer adhesion and no extruder skips.
All that with an otherwise box-stock 12 volt PSU-equipped FT-5 R1.
And without hovering over it holding a fire extinguisher or having it walk itself off a table top as it runs.
However that has not been my experience with it.
Still, I am very happy that I bought my FT-5 over the Tevo Black Widow, the closest contender at the time.