US airport websites often show gate info way in advance (sometimes over a day) and then have to change it, but it is still useful. At an airport like LHR the gate may genuinely not be known until the income aircraft is allocated it on short finals.
One problem is that the philosophy of computer systems generally seems to have changed over the years. You used to be able to get a lot of information but now it is all "personalised" you can get it for an individual flight but not an overview. I miss not having an overview - a quick scan often gives an idea of whether there are general ongoing delays. At outstations you could check if the inbound service was on time. (This philosphy is an even bigger problem with timetables, indeed a right pain in the derriere if you want to plan a multi trip itinerary.)
A multiplicity of codeshares has created problems with some websites although that is easily solvable - have a show codeshares or not button.
Flightstats was very good. They now charge for the full service but you can still get data for the day of departure for free.