Like Kira, I've had the opposite experience.
Vr2 has a too-flexible mid-section, but it was strong. Vr6 is POWERFUL. No way is it weaker than any other internal vibrator.
By contrast, Lelo Ina and Elise are weak and disappointing. Siri is too weak for me too. I like Gigi, probably only because its shape alone is fantastic, so any vibration it offers is icing on the cake.
Maybe the difference is whether you prefer buzzy (Lelo) or deep (Jopen) vibrations.