Well, the compendium said that VF-171 is easy to use, maybe not as powerfull as the VF-19/22, but for a mass produced grunt valk, I prefer a valk that is easily replaced while pilot training is faster (no need for Basara or Isamu type to handle the valk).
With the introduction of VF-25/27, I say the next generation valk is the VF-25 while VF-27 is reserved for assault valk. VF-25 have all the characteristic of main force valk (several variant like squad commander, etc, and also can be equipped with FAST pack and Armoured pack).
Not to mention the introduction of EX-Gear system mean the pilot could be handle the high-G performance of the new valk relatively easier compare with other model.
On side note, NUNS decide not to adopt VF-19 as main fighter after they found out that most VF-19 stationed on some planet are strangely painted in red and came equipped with speakers and later, stolen by Basara. J/K