My partner and I have miniature dachshund puppiess (one each). I live in a house, he lives in a flat - so both our puppies are used to spending huge amounts of time at either property.
On paper, everyone told us that my house with garden was a much better bet for raising dogs than his top floor penthouse apartment with only balconies. In reality, the small rooms of my old house means that when my young kids are home and the puppies get the zoomies its absolutely chaotic, and feels like there's never any room to move without tripping over each other. In his big open plan apartment the puppies wear themselves out with the freedom to run around a space more vast than even my garden, and they are never in our way. They have coped absolutely fine with no access to a garden - from day one we taught them to ring bells hung on the handles of the patio doors leading to the balcony - and we use a small rectangle of astro turf in the corner which they know to go on. Its small enough that it can be regularly washed and replaced so it never gets smelly, and the dogs have both been absolutely fine with using that so the remainder of the balcony remains clean for us to use. Doing it that way also hasn't in any way confused them with going on 'real' grass when at my house or out on a walk, they just know that its different scenarios at different places and roll with it.
It does obviously entirely depend on your set-up, and the layout of your flat... but from my experience of raising puppies in both settings simultaneously, id say in our case the flat was by far the easier option actually. It helped being an open plan flat so dogs were always in sight (ideal for toilet training as we were never inconveniently in another room when they started circling to pee) The whole toilet training thing does seem daunting at first, but the reality of it is that stage only lasts anywhere between a few weeks to a few months at most - and then you have grown up dogs that can wait to go out for a walk, and at worst only need a bedtime wee or quick access if they are poorly. Its not worth deciding against doing something for a slightly inconvenient few weeks... cos having a puppy is hard work for those weeks anyway wherever you are!. You have to very much judge it on your own set-up and what you think can make work, not make a decision based on what looks ideal on paper.