Well considering the champions league is the bread and butter for that level then yes he's under achieved. Could say Ferguson definitely did in that respect. Look at ancelotti he's won it how many times with how many clubs, he literally goes in to do win it
Ancellotti is the ONLY manager to have won more Champions League trophies than Guardiola? Does that mean everyone apart from Ancelotti has underachieved? It's hard enough to win it once, let alone multiple times.