Similar word to upstream:

Gargantuan, Herculean, ambitious, arduous, backbreaker, bothersome, burdensome, challenging, crucial, demanding, difficile, easier said than done, effortful, exacting, formidable, galling, hard-won, heavy, immense, intricate, irritating, labored, laborious, no picnic, not easy, onerous, operose, painful, problem, problematic, prohibitive, rigid, severe, stiff, strenuous, titanic, toilsome, tough, troublesome, trying, unyielding, uphill, upstream, wearisome