[mjn]

That's a reasonable criticism, but I think quite different from ideological extremism. Their problem, at least at the level of the leadership, is more just populism: they are telling Greeks many things that Greeks want to hear, which are mutually inconsistent. If anything the leadership lacks much of a hard ideological conviction, mixing together an eclectic combination of leftish economic sentiment with rightish nationalist sentiment, and a kind of paradoxical pro-EU/pro-euro position fitting uncomfortably with both of those. Hence there is this weird mixture of demanding German war reparations on the one hand (a completely counterproductive negotiating tactic purely for domestic nationalist consumption), trying to renegotiate the debt in a fairly technocratic/moderate way (into GDP-linked bonds) on the 2nd hand, a weirdly brash personal style on the 3rd hand which works badly with #2, a "red line" of no exiting the Euro on the 4th hand which restricts options considerably but is popular in Greece, and a bunch of electoral promises of the usual bread-and-wages kind on the 5th hand, which the combination of positions 1-4 leaves them virtually no room to actually enact. This all works very well for domestic politics, and the Syriza–ANEL coalition, a coalition of a large left-wing populist and a smaller right-wing populist party, is actually considerably more popular now than when they were elected. But it's not clear it is a winning strategy.

The actual left of Syriza has become very critical internally over that line, which they see as incoherent and not in tune with reality: in their view, the line Tsipras/Varoufakis are attempting to take just won't work, because it wants to promise staying in the Euro and then also a bunch of things that are incompatible with staying in the Euro, but won't admit that you need to pick one. Here is one interview with a left-wing Syriza MP, criticizing them along those lines: https://www.jacobinmag.com/2015/03/lapavitsas-varoufakis-gre.... You can differ over whether this line would be better, but I think it's more grounded in reality; Lapavitsas (the interview subject) imo much more clearly understands the concrete situation and what can and can't be done, and doesn't promise glibly that everything is simultaneously possible.