Of course they'll be disappointing, as long as they beat the other new teams.
They'll only be disappointing if they fall short of expectations, no one is likely to be expecting them to beat Brawn/RB/McLaren/Ferrari, so they won't be disappointed when they don't, but like you said, as long as they beat the other new teams (or any other Cossie engined car) they'll have done well, hopefully will stay in the sport and improve as the years go on.
They'll only be disappointing if they fall short of expectations, no one is likely to be expecting them to beat Brawn/RB/McLaren/Ferrari, so they won't be disappointed when they don't, but like you said, as long as they beat the other new teams (or any other Cossie engined car) they'll have done well, hopefully will stay in the sport and improve as the years go on.