In my experience it only works if you pull in juniors into strong teams and keep the proportion of juniors reasonably small. You also need to have a process in place for training them - it’s not enough to rely on ad-hoc mentoring from peers.

If you get the ratio wrong, with too many juniors and weak technical leadership then you will end up in a very bad place in your code base.

In terms of value, even if juniors are half the cost, it is much wiser to hire one senior instead of 2 juniors for the same money.