Umm, low awards have always been capacity controlled, and you'll find that on any airline. US doesn't say that every seat will be available for low award redemption. If you want that, then pony up for a standard award. If US thinks they can sell a seat, they're not going to give it away. That's true of upgrades as well.
US tends to be a bit stingy with Envoy awards, but keep in mind you're also looking during a peak travel season too. You're not the only one looking for those seats.
Take a look at Star Alliance awards. You can do this on United's or Air Canada's websites and usually you can find what you're looking for if you're flexible. I booked quite a complex itinerary in C for July (actually July 5 - right during a holiday) for SAN-PHX-ORD-WAW-VIE-PRG-VIE-ORD-BWI. That's a mixture of US, LOT, and Austrian. UA and AC had a decent amount of availability to Europe in the summer - ironically it was the domestic F seats that were hard to come by (US had F, UA didn't).
Good luck.