Most people accept the extra work needed to get a daylily or an azalea off to a good start, the extra watering, extra fertilizer, extra attention. But a new bed itself has needs that diminish as it gets older.
Weeds, of course, pop up frequently. No matter how much you mulch, a few always get through. But few people realize that, if they get the weeds out right away, the next year there are fewer. And the year after that hardly any at all.
The first few years, despite well-designed plans, something always gets larger than you expected, or comes up the wrong color, or just plain fails. So you buy something else, move things around, change your mind. All very well, but every time you expose bare soil, the weeds get a chance to thrive. But, eventually, everything settles down.
And then there are ground covers. They can be disasters if not weeded until they're thick, but they also stabilize a bed and, eventually, keep the weeds away. If possible, plant starts close together and mulch well. Keep an eye on them and get the weeds out right away. Tell yourself this is only a couple years of work.
So, with all this time and attention needed, starting small is a good idea. I once helped an excellent gardener and rare plant fanatic who started one new bed a year. If you have the patience and the opportunity that's the best way to do it.