Some, but def. not "most". In fact, had some serious issues with online changes recently - even with UA (which imo has some of the best IT).

In this recent case it was Air Canada and their online services are pathetic (and customer service completely unreachable now).

Whenever I need to book airlines with iffy customer service, in particular for tickets that are supposed to be refundable, I usually go to Expedia. Just did that for Royal Jordanian tickets. For something like UA - I'd book directly,

P.S. Expedia also has a better 24 hour cancelation policy (in fact, if you book before a weekend - almost 2 days), for most tickets (where airline itself would not).