The main reason for not using <br> is that it’s not semantic. If you want two items in different visual blocks, you probably want them in different logical blocks.
In most cases this means just using different elements, for example <p>Stuff</p><p>Other stuff</p>, and then using CSS to space the blocks out properly.
There are cases where <br> is semantically valid, i.e. cases where the line break is part of the data you’re sending. This is really only limited to 2 use cases – poetry and mailing addresses.