It isn't easy to see what's wrong with American Visions (Sun BBC2), Robert Hughes's grand account of American art, mostly because of what's right with it. It's impossible, for example, for anything involving this master of intellectual plain-speaking to be simply dutiful - even if the very nature of the enterprise forces on him some duties he might not have volunteered for. (This is not solely Hughes's America, after all, it is Time-Life's and the BBC's, so there will have to be excursions to sites which may hold no great charms for the guide, moments when his manner takes on the constraint of politeness.) It's also virtually impossible for the film to look bad - there is always the American landscape to fall back on, both a guarantor of what has been made of it artistically and a respite from the diminished presence of paint seen on television. In the soaring helicopter shots which adorn the series you have, for me anyway, an unfailing resource of exhilaration and pleasure. Nor is the series too piously narrow in its material - discussing the American landscape tradition a few weeks ago, Hughes traced the sacramental worship of nature to its absurd present-day conclusion - a group of Earth Firsters sobbing in the forest, apologising to the trees for mankind's lack of consideration. If trees actually did have feelings, they would almost certainly have fallen on them to stop their caterwauling.

And yet, at the end of every programme, there's the faintest sense of something missing. I would venture two possibilities. The first is that Hughes's sharp talent for the superlative has been blunted on one of its edges. So that while he is still able to deploy a recruiting trumpet-blast for the art he loves (last night he convinced you to fall in love with Falling Water, Frank Lloyd Wright's astonishing cantilevered house), the art he doesn't gets away relatively unscarred. Discussing Georgia O'Keeffe, for example, he was equivocal about her talents, hinting that her fame was a result of gender politics as much as her own achievements. But he finished by describing her as a "very considerable painter", an unusually inert phrase for such a muscular writer. I may be imagining it, but something in his voice suggested he didn't really believe this - that O' Keeffe was present because no comprehensive endeavour could afford to leave her out.

Which leads on to the second possibility - that the overwhelming problem of all such projects (the battle between inclusiveness and coherence) hasn't been quite resolved here. American Visions sensibly orders its material by theme, rather than chronology, which means that each episode can have its own shape and can also avoid a monotony of style. But it can also sometimes seem that the thematic connections are wearing a little thin. In last night's programme about the influence of immigration on American art, Frank Lloyd Wright's Robie House appeared because of the Japanese influences on its interior. But importation is not really the same thing as immigration - it was as though a place had to be found for this precious object somewhere, as though the cataloguing instinct had won out over the argumentative one. Other passages struck you as being a continuation of earlier programmes rather than this one, as if Hughes had found a glut in one thematic area succeeded by relative scarcity in another. None of this, incidentally, argues against watching these programmes - Hughes slightly off his best is still better than most and there are reasons to believe he will hone his blade as he comes closer to the present day.

TX's programme (Sat BBC2) about Eddie Izzard's world tour conveyed something of his weird courage (watching him struggle through a set in French, the phrase that came first to mind was "mourir comme un chien" - his style of surreal comedy requires an absolute confidence that the performer means what he says and isn't just having vocab problems) and the brilliance of his material. But it may have been unwise to include so many shots of aeroplanes taking off and landing in a programme that never quite managed to leave the runway itself.