The hardest thing for a spouse to adjust to is a rotating schedule, working nights/weekends/holidays, being held over after the end of your shift whether it's for a few hours or 12-24+. Some employers will also wipe the slate clean and change everyone's schedule every few years, or every 6 months in the case of CCEMS.
It's difficult to make plans after work due to late calls and holdover, and some employers give you leave but make it prohibitively difficult to actually use it. Being held after work for coverage or late calls can interfere with child care as well.
The worst schedule you can work is a 24/48 with no kelly's (extra day off every three cycles), especially in a busy system. You work one day, run calls the entire day, get little to no rest, and then need to spend the first day off to recover. that leaves you with only one day for family time where you're 100%. It always feels like you're going back to work the next day. Now add holdover or a side job (EMS pays OK in some places, but you won't do well unless you work extra hours) and you're never home in a rested state.
Some places make it difficult to take leave. One place I worked for required you to get someone to cover your shift for OT before you would get approved. If you work 24's, you can't even plan the start of your vacation that morning because you risk holdover.
Your spouse may not be able to adjust to you being out of the house more nights than you're home. If you've had a particularly tough day, or stressful call, your spouse may not be able to relate or give any positive feedback.
In my case, my wife became pregnant while I was a basic. I chose medic school over RN school as it was quicker, cheaper, decent pay after graduation, and there was still the option afterward to go to RN school either online or in the traditional sense. I worked my scheduled 40 hours, some OT, and spent an average of 32-36 hours weekly between medic class/clinicals for 13 months. This was while my wife was taking care of our infant daughter, often alone for many days and nights. She harbored deep resentment towards me for that, up until I landed my current firemedic job. Life is good now, low financial stress, great work schedule. she understands why I did what I did now, finally.
After graduating medic school, I upgraded to medic at my job and secured a couple of per diem medic gigs at local hospitals as well. Like many other areas, medic money is okay, you'll stay fed, the bills will probably be paid, but you'll never be more than a paycheck away from going into debt. This financial stress can put strain on a marriage "You went to school for all that time, you weren't around AT ALL for over a year, and we're still poor! what good did that do? Now we're 5 grand in debt!" I did what many in EMS do. I worked side jobs. All the hours they would give me. It's blood money, but you'll be able to save for a house, car, college for your children, etc. Medic money is good enough to stay fed, but certainly not enough to prosper. You can make enough as some educated white collar professionals, only if you're willing to work 60-80 hours per week.
That's what I call the medic "black hole". You can live on a medic salary, the money's just good enough for you to do okay with OT and side work. Just good enough that you'll accept that as your reality from now on instead of going back to school for a legitimate degree and a better living. It's more than difficult to go back to school whan you have a family, and need the money from both FT and PT work to survive. You're now stuck for life, basically. You'll work crazy hours and miss countless family/social events until you retire, hopefully with a pension and not a 401k that you can't afford to contribute to anyway.
This is what causes cumulative stress to the relationship. "You can't make Johnny's graduation?" "You're stuck at work again? Okay, I guess we're eating alone as usual." "Why can't we afford a house with all of these hours you work?"