Thinking about the situation again, I believe that perhaps the reason why so many people believe their marriage was always a struggle was because when it came to the show giving the viewer the issues relating to relationships and the morals that come with them, they always used the O'Briens as the whipping mule for that stuff, thus we were exposed to more problems and issues between them then we were exposed to them being happy and working together.
They sorta ruined the balance with them and perhaps should have shifted some of the relationship stuff off to Sisko and Yates once in a while, whom were usually given the lovey-dovy scenes and being nice to one another......
.... having Yates and Sisko (for the most part) being happy and helping one another out all the time and being affectionate and then having the O'Briens usually dealing with the conflicts, troubles of work and family, and all the quarrels that come with relationships/marriage...... it's easy to fall into the thinking that their marriage wasn't a very healthy one.
But there were times where they showed that not only did they really care/love each other, but they knew each other inside and out...... like the time when Keiko knew Miles wasn't her husband but someone else due to him drinking his coffee later in the day which normally keeps him up all night "
Armageddon Game":
"The next day, Keiko comes up and asks to see Commander Sisko. She shows him, Kira, and Dax a part of the recording where O'Brien drinking coffee. According to the time index, it is late in the afternoon, and she says he never drinks coffee in the afternoon, because it keeps him up all night. She is convinced the footage has been altered. Sisko isn't sure, but he joins Lieutenant Dax to get the Ganges a day early, just to see what's going on."
Of course in the ending of the episode, he corrects her, saying that he occasionally drinks coffee later in the day, which made her own knowledge of her own husband not only incorrect, but a complete fluke that she cracked the conspiracy wide open and still saved him and Bashir.
But her devotion to not accepting that her husband was dead and how both know each others little quirks (mostly, lol) shows that they do still care/love each other.
Granted my marriage hasn't had as many fights/quarrels as their marriage, but they exist from time to time in any relationship..... the willingness to work through problems and to not give up on each other is proof that they care enough for each other and their family.
If they didn't, then they wouldn't work on these things, give up and just let the whole thing collapse.
In regards to their different careers, Keiko for the longest time backed and supported Miles' career/dreams of being in Starfleet, tinkering with machines and being needed by many..... and when it came to her finally getting fed up with trying to make a life on DS9 and headed to Bajor for a while, It was only fair for Miles to support her in what she wanted to do... relationships have to be a two-way street and if your partner allows and helps you out on obtaining your dreams or needs in life, then it's only fair that if you really care and love your partner who's supported you, that you help them and support them in their dreams/needs in life..... even if they don't really appeal to you.