The game already has a mixture of VR and non-VR players. I don't get your point.
Sure. But currently we all use the same inputs (K/M / controller / HOTAS). FPS would likely introduce motion controllers, and suddenly you've got much more disparate systems. (As a brief example, the fact that VR players could blindfire around corners easily, whereas flat gamers would need to have that ability added via animations / keybinds etc, if they sought parity there).
They could easily just not bother too much about imbalance on the PvP front. But I suspect they'll want to at least take a token pass. And there's also overlap with other design best practice. (IE 'can a VR player shove his head through a wall' doesn't just have implications for 'wall hacking'

)
If VR players are going to get nausea they already know it. They will already have experienced every kind of locomotion in VR if they have played long enough.
I guess I'm saying two main things on this aspect:
A) FDev are focusing this DLC on inviting a new influx of players in. If/when they add VR, they will want to make sure it has its noob-welcoming front foot forward. So that means best practice in terms of nausea reduction too.
B) There are slightly different nausea issues at play on foot. (In the ships we don't experience fast yaw turns or accelerations for example, that classic nausea trigger, but in foot gameplay it's a staple). Although there are lots of decent solutions to that now (controller-relative motion, HMD-relative etc), they'll probably still hit up against design conflicts when trying to design systems that work for both flat and VR. The jetpack aspect is one that occurs. (Long acceleration curves might feel cooler, and have gameplay utility for the designers getting the feel of the jetpack right, but are also a nausea trigger in VR etc. Any jetpack system will already have to work with the various gravities, so this type of aspect is just adding constraints on top of constraints).
None of it's insurmountable. They can just work their way through it. They could just have distinct systems for flat & VR. (Say a jump pack that uses long acceleration curves for flat, but short, sharp ones for VR). Presuming that still works for any other factors. (IE design wanting jet-packers to be able to reach certain rooves, or whatever)
---
What I'm saying is overall is: There are a lot of aspects to ED that make FPS VR non-trivial to add. If they want to do a half-decent job.