70d is way stronger than 30d, not sure why it's underused.
Things I so far found it good for: the rolltop of a backpack, perfect weight and strength.
Prototyping backpack, or for ultralight pack, side and front panels.
Good weight for water bottle holder I found.
It's a bit heavier than tyvek housewrap when PU, but a better material I'd say overall for ground covers if you're not going super light with polycrio type stuff, also depends on how thick the pu coating is of course, you can feel that easily though.
I bought a bunch too from diy and oware, mostly for prototyping, but also I am using it for some of my pack parts, the channels for frame tubing for example, it's just right for that, if you reinforce the two ends with heavier nylon.
If I was going to make a serious attempt at a tent, I think I would be very well advised to make a full scale functional mockup with 70d first, that way I'd have a strong durable and totally waterproof tent while working out the design glitches. PU is about 2.2 oz / yd, good quality silnylon by my scales comes in at 1.6, the 1.35 oz stuff I assume is what shield type sil is NOT. That's only .6oz yd difference, that's really not very much. Maybe .7, I'd have to weigh a piece of shield to see for sure. I know all the thicker 30d sils I've bought weigh 1.6 oz/yd.
A poncho too, I think one reason some people don't like them much is that they made them out of silnylon, which as noted here on bpl forums, over time starts to break down and lose its water resistance, plus sil is going to be so light and flappy in wind it would be annoying. I know my old 70d poncho I bought and which is still pretty much totally waterproof wasn't that bad in the rain and wind.
But I think prototyping is the best, I made a rough mockup of a backpack to test various things all out of 70d, cheapo, and it cost me basically nothing, and helped solve some weird glitches in the design, and 70d is just strong enough to act like the thicker nylons more or less.
In fact, I liked the prototyping method so much I decided to get more coated 70 and 200d nylon from diy and other places so that my prototypes would be fully functional packs, I find it hard to motivate to make something I'll just throw away. I was using the very ugly and barely coated orange nylon that oware I think sold some time back for almost nothing, but that is really just good for raw prototyping.