stressful

bruising, burdensome, exhausting, labored, onerous, oppressive, taxing, tight, trying

arduous, Augean, backbreaking, challenging, demanding, difficult, effortful, exacting, formidable, grueling (or gruelling), hard, heavy, hellacious, herculean, killer, laborious, moiling, murderous, pick-and-shovel, rigorous, rough, rugged, severe, stiff, strenuous, sweaty, tall, testing, toilsome, tough, uphill

abstract, abstruse, complex, complicated, elusive, hairy, insoluble, intricate, involved, knotty, opaque, problematic (also problematical), recondite, serious, spiny, stubborn, thorny, ticklish, tricky

annoying, bothersome, distressing, irksome, troublesome, vexatious

grievous, grim, strict, stringent

brutal, cruel, inhuman, painful