It’s a bit like Dehaka’s Broodbrother (P3) in a way, in that there’s (essentially) no downside, but it’s up to you how much of an advantage you make of it.
Your first Shadow Guard will only last 40 sec instead of 60, but the moment you get Time Stop out you’ll get an extra free 40 sec. So here’s your Shadow Guard uptime in the early game:
| Prestige | Mastery? | Duration | First Use | Free Use | Uptime 3–6 |
|---|---|---|---|---|---|
| Base | No | 60 | 3:00–4:00 | N/A | 33% |
| P3 | No | 36 | 3:00–3:36 | 4:00–4:36 | 40% |
| Base | Yes | 120 | 3:00–5:00 | N/A | 67% |
| P3 | Yes | 72 | 3:00–4:12 | 4:12–5:24 | 80% |
80% uptime is pretty amazing. At that point you can forget about Keeper of Shadows and just play normally if you like, but with 4 Shadow Guard in each Time Stop. To get more out of it, you will be making a trade: every time you cast Shadow Guard in the future, you lose out on Black Holes and Dark Pylons. If I remember rightly, it takes her 3 minutes to generate 50 energy, which is the Shadow Guard cooldown. So if you go ‘all-in’ you’ll only be casting Shadow Guard.
Is that a good idea? Maybe not, but you can choose to cast it rarely or never and you’re still getting something.