Universal is an old term which is still part of the legislation and basically means that it fits with a seatbelt as opposed to having to be bolted into the car.
The Maxi Cosi Nomad is actually less compatible than a lot of other seats - it's just marketing that they say that.
MOST seatbelt fitted car seats fit into the vast majority of cars. Could you say a bit more about what problems you've had so far?
Do you know your son's height? 12kg at 17 months is on the bigger side, so you will need to be a bit more careful choosing seats compared to average. For example, "Up to age 4" will likely fit him until about 3.5 years and "up to age 7" will likely fit him until about 6. If he is tall for his age, these ages might be younger. It's close to a border so you might be a good contender for some of the "wiggle room" seats where the weight OR height allowance is very slightly bigger than the usual while the other is standard.
Useful to know: You're looking at a rear/forward facing harnessed stage currently. Most seats "Up to age 12" are actually an "up to age 4" harnessed stage, with a booster mode added on, and some "Up to age 7" are as well (just with a shorter-lasting booster). Booster seats are the most compact and much easier to move around than the harnessed stage, so I would forget about buying a seat with booster mode built in - this won't save you hassle in the long run. In reality, as a non driver, you will want to ditch this heavy, awkward, bulky stage ASAP and swap to a lightweight, possibly folding high backed booster. You don't want a booster (later) with all the annoying bulk and weight of the harnessed stage.
Do you have a budget in mind?
Do you often travel in cars without isofix? These days the vast majority of cars on the road in the UK do have isofix, in 2006 about 75% of new cars had it, and it's been required in all new vehicles since 2013. So unless someone you regularly travel with has a very old car, or you often travel to countries where it's common to drive older vehicles, or in black cabs (which I think don't have isofix?) then you might be fine with an isofix seat, which may help as these can be a bit more universal.
What is your normal scenario for car use - do you tend to get lifts with the same person, or are there a small number of cars you regularly travel in, or is it much more varied? Do you commonly travel with people who might already have a car seat, or do you usually need to provide your own?
When you do get a lift, do you get picked up from home, so you can take your time to install the seat and it doesn't matter if it's heavy or bulky, or do you need to transport the seat to the car, or carry it around with you when you reach your destination?
It is a tricky stage for car seats and lifts. So working out what your actual needs are helps a lot.