Just because a website says a plant should do well in a given area doesn't mean that is necessarily will. All information must be taken with a grain of salt and then verified before one invests too much.

I'm sure the original design intent is that the junipers grow together and be the "groundcover." Using mass shrubs as groundcover has the disadvantage that it's difficult to limit their height without mucho maintenance. What inevitably ends up happening is that they become too large and unwieldy, make somebody unhappy and then get ripped out and replaced. Adding another groundcover in between them would look weedy. Though I would do this if you intended to replace them with that other groundcover, and remove the juniper as the other groundcover took hold.

Another disadvantage to the sea of junipers is that you really can't add trees (though it looks like some might be useful for screening) because of the shade they produce. Plants that demand full sun

It would help on the weeds if you used Preen, a pre-emergent herbicide that attacks germinating seeds, and mulch heavily. Many people give up on Preen because it usually doesn't do a great job until after its second application. And also, because they don't adhere to the schedule, putting the second app. on way too late so that it becomes just another "first" application. I have found it to be effective and basically a life-saver. It can cost a fair amount to cover a large area, but in my opinion it still beats the total cost of hand weeding once the time and PITA factor is added up.