Yes it is, as tough as any other marriage. I have faced this question so many times over the past years, and each time I have answered differently, and only now, I believe I have my final answer with me. Any marriage is equally easy or difficult, whether it is inter-caste/religion/region/nation or intra-caste/religion/region/nation. What counts are the people involved.
A marriage if seen from the Indian perspective, of uniting families, is always a lot tougher than one, where just two people commit to each other. The more the people, the more the likelihood of trouble, because there is greater amount of interaction , with greater likelihood of frictions. If the question is subjected to marriage as the relationship between just a man and a woman, I would say whatever their cultures, whether the same or varying, making a marriage succeed or fail is completely about the two people involved in it. Their level of commitment to the relationship, their belief in each other, as well as the mutual love and respect. A marriage works or does not, just based on the people involved, how well they get along and how deep is their affection for each other. Their region, religion have no consequence in a relationship that close.
Couples fight no matter whether they belong to the same culture or not, but what they fight about, how they handle their differences is completely based on them as persons and nothing else. It is unfair to think that belonging to the same culture would mean less marital discord. How does something like religion matter when you are sleeping with a person, living day in and day out, even having babies together? But if they do split up, the cultural difference is just an excuse to put up. Would they not break up if they belonged to the same faith? I don't think so, the difference in region or religion can only be an add on, or an excuse for breaking of a relationship, in which the reason lies much deeper. I have friends married to people of the same subset of background they belong to, with enough marital woes to last them a lifetime, and there are those married to people as different from themselves as can be, and yet their marriage rocks. This is just an example to say that the typical mindset of marrying a person of a different background spells doom is not correct, because every marriage is eventually the product of a relationship between two people, and just the two people alone.
A regular married couple can fight about who gets up in the morning to make tea, but I doubt that it is because of what religion they belong to, they may have altercations about their children's education, but does it really matter here, what their caste is? I think any marriage, is a reflection of the people who make it, it can be a sweet loving marriage between two people from different parts of the world, or the most horrible suffocating one between two people who share a common heritage. This post at the Indian Homemaker's blog for example, is about a woman in a hopeless situation, how would it be different if either of their religions, caste or creed varied? At the end of the day it is about the two people involved.
Coming to the bigger perspective of an Indian marriage which involves families, the reasoning remains the same. It is about the people. A nice person would be nice irrespective of their background, and a nasty person would be they way he is. In such situations, the difference if present, is just that another tool is used to hurt or taunt. In my case for example, if there have been relatives from the in-laws side who have passed snide remarks about my being from a different faith, I know for sure, that those particular people would have passed snide remarks irrespective of my faith, just that then the context would have been different. Wouldn't you agree? When we think of relatives, some people are always out there to get you, and after marriage it is quite impossible to completely evade the politics, and games that exist in a scheme with a large number of people involved. Some have personal vendetta or ulterior motives, some are plainly hurtful, some will befriend you defying all expectations, some just sit quietly and observe, there those who will stab you on the back, and also those who will stand up for you in your absence. There are all kinds, everywhere.
At the end of the day, it is all about the people, their personalities, and their values, which do or do not create trouble in a marriage. The commonality or difference in cultural backgrounds are just a very small, almost insignificant part of a marriage, the big things are trust, love, mutual respect, the want to create happiness, honesty and ofcourse the readiness to change.