I feel so too. Relationships need to be based in deeper God consciousness to harmonize and blossom. So in that sense both parties being of the same spiritual group would be a good start.

Even if they both align to one group, their individualities will remain after formal marriage...and the deepening will still need to be worked at.

Throughout the christian community back in the 60's and 70's (in my parents time) it was necessary for a protestant to convert to marry a catholic. I believe these things are not the main issue of spiritual substance. What to say of different Gaudiya groups being unable to harmonize. This Gaudiya Math/Iskcon thing surely should not come before love (I am not talking about lust here). And if it does, in my opinion, there must be some incorrect spiritual teaching in one of the groups involved.

Ofcourse though, it depends on the spiritual maturity of the couple. And maybe that is why some leaders try and protect their flock (from the slow spiritual journey plagued by doubts).