My understanding is that UA will book SQ (if available to *A - e.g. as shown on the Aeroplan web site) if you call them. Also UA still permits stopovers on return trips. The miles are more expensive than LM even with 80% bonus (which is a targeted offer and not offered very often) and UA's booking window is only 334 days even if the *A airline you wish to book has a window to 354 days (as does AC, OZ, NH, LX, LH, ..).
I think it's harder to accumulate large numbers of miles due to per purchase and annual limits; you also cannot buy additional at time of booking like LM.
Personally with the AUD in the low 70ies I find the whole value proposition of buying miles not very attractive any more. I've stopped buying and will run down my stockpile of AA, LM and SQ then look for paid deals on business class - e.g. right now SYD-LAX return in business is as low as $AUD 4500 during July and August - SYD-Europe can be as low as $AUD 5500 return in business on one of the AY or LH deals.
