This ^^^^^
If I do 30/15s at same peak power as 30/30s I’ll hit 90-92% max HR during the sets and it pretty much looks like a continuous interval if you look at heart rate trace after. With 30/30s I recover pretty well between the on intervals and the heart rate doesn’t climb above 85% and trace looks more saw tooth.
Just to state the option, if people want to use ERG but want lower power targets, reducing the Workout Intensity setting is there to use. Lowering it will reduce the power target in ERG, akin to what can be done in Res/Std modes and missing the target.
It still holds you to the reduced power and can lead to the death spiral if you are on the edge. So Res/Std still has plenty of reasons to be used for stuff like this, but ERG is not just locked at 100% as implied in many cases. Drop that WI setting and carry on in ERG if that’s your preference.
So doing it in resistance mode IMHO opinion is better for a few reasons.
Spiral of death is the worse, and when it comes mid-VO2 it is terrible.
If the goal is maximum oxygen uptake, say you went out hard at the beginning, but are fading at the end, even with “threshold watts” you probably struggling to hold on and eliciting the requisite stress on the body.
You don’t want ERG mode to dictate how hard you are going. If you are really fresh, it might take going harder than the target watts to elicit the proper response. Sometimes, it might take less than the target watts. Either way, the point is to tax the system rather than hit a certain number. The target watts may be a good reference point, especially for someone who has never done it before, but ultimately VO2 should be as hard as you can go for the prescribed sets.
Adding onto the reply, it doesn’t hurt to do both micro-burst, and longer sustained efforts. Typically when I am focusing on VO2, I will do both types of workouts in a given week. Even though I personally may have more overall time spent near maximum oxygen uptake by performing micro bursts, those longer sustained intervals tend to be more like scenarios in races when trying to bridge to a breakaway, or take a late race flyer!
VO2 and power targets are only a suggestion. For any VO2max interval/ repetition, you go as hard as you can for the duration with a constant power. if with 30/30’s for you 125% does not get you in that “out of breath” breathing rate, you need to go harder.
If I had to guess it’d be neuromuscularly. It’ll be easier to churn out big watts because your brain can recruit all the muscles, but the growth in the anaerobic stores or aerobic system still takes much longer.
My favorite VO2 workouts are the billats workouts. But I ride it on resistance mode and will do a quick 30 second surge before the work intervals to get HR up then pop it into erg. Works nicely 
I just did Joe Devel +3 and for an anaerobic workout, I got my HR awfully high during 45/15s and the 30/15s but I was on the road and found it a lot easier to tease out higher watts on the “on” segments.
Do Joe Devel but out on the road. That’ll get your heart rate up!