Yeah, it's true, the AP website isn't great for multi-city awards (which have more than 1 stopover / 1 open jaw -- you can call to add the second). The United site is worlds better than the AC site for booking awards. But you know what? The US Airways site is far far worse because it doesn't show partners, so thank goodness for small blessings And the AC reps are both friendly AND knowledgeable! $35 is a small price to pay for their assistance is booking complex tickets imho. Now YQ is another matter.

As for your complaint about the AC site, you can't really use the AC site to book flights within the US, because the AC website will only show you AC and AC codeshare flights. Because AC isn't really a travel agency they will only issue tickets on 014 (AC) stock and with AC fare basis codes. If your travel is primarily within the US on routes AC doesn't publish fares for (i.e. flights operated wholly within the US by United or US Airways) then AC will not be able to issue you a corresponding ticket on 014 stock with an AC basis.

If you're traveling primarily within a country other than Canada, you cannot use the AC website -- or AC reservations by phone for that matter. You would be best served with a travel agent, the UA website or an OTA such as Expedia, who can issue tickets on non-AC stock.

TL;DR: Yeah AP.com could be better, but it's not half bad. For a long time it was one of the best. Now it's middle of the pack. AC.com is pretty much exactly what it was designed to be. I'd be thrilled if they allowed booking non-codeshares for connections in Europe and Asia, but reservations will do that for you by phone, free of charge