To give more options to the already suggested ideas, let's dig up Oxford Thesaurus of English.
he was unable to follow the abstruse arguments put
forward: obscure, arcane, esoteric, little known, recherché, rarefied,
recondite, difficult, hard, puzzling, perplexing, enigmatic,
inscrutable, cryptic, Delphic, complex, complicated, involved,
over/above one's head, incomprehensible, unfathomable, impenetrable,
mysterious; rare involute, involuted. ANTONYMS clear, obvious.
an extraordinarily convoluted narrative: complicated, complex, involved, intricate, elaborate, impenetrable, serpentine,
labyrinthine, tortuous, tangled, Byzantine, Daedalian, Gordian;
confused, confusing, bewildering, baffling, puzzling, perplexing;
informal fiddly, plotty; rare involute. ANTONYMS simple,
intricate Arabic patterns | the intricate
relationships between plants and animals: complex, complicated,
convoluted, tangled, entangled, ravelled, twisted, knotty, maze-like,
labyrinthine, winding, serpentine, circuitous, sinuous; elaborate,
ornate, detailed; Byzantine, Daedalian, Gordian, involved, mixed up,
difficult, hard; informal fiddly; rare involute, involuted.
ANTONYMS simple, straightforward.
Taking simpler options:
the complicated election rules: complex,
intricate, involved, convoluted, tangled, elaborate, impenetrable,
knotty, tricky, thorny, serpentine, labyrinthine, tortuous,
cumbersome, Byzantine, Daedalian, Gordian; confused, confusing,
bewildering, baffling, puzzling, perplexing, difficult to understand,
above one's head; informal fiddly; rare involute, involuted. ANTONYMS
easy, simple, straightforward.
1 a complex situation | criminal law is an extremely complex
subject: complicated, involved, intricate, convoluted, tangled,
elaborate, serpentine, labyrinthine, tortuous, impenetrable,
Byzantine, Daedalian, Gordian; difficult, hard, knotty, tricky,
thorny, problematical; informal fiddly; rare involute, involuted.
ANTONYMS simple, straightforward. 2 a complex structure: compound,
composite, compounded, multiplex.
They are all suggesting similar words, but there are some fine differences. Lastly, let's switch sides!
1 the process was remarkably straightforward: uncomplicated, simple,
easy, effortless, painless, undemanding, unexacting; elementary, plain
sailing, a five-finger exercise, child's play; routine; informal as
easy as falling off a log, as easy as pie, as easy as ABC, a piece of
cake, a cinch, a snip, easy-peasy, no sweat, a doddle, money for old
rope, money for jam, kids' stuff, a breeze, a doss, a cakewalk; N.
Amer. informal duck soup, a snap; Austral./NZ informal a bludge, a
snack; S. African informal a piece of old tackle. ANTONYMS
2 a straightforward man: honest, frank, candid, open, truthful,
sincere, on the level, honest-to-goodness; forthright, plain-speaking,
direct, unambiguous, straight from the shoulder, downright, not afraid
to call a spade a spade; informal upfront, on the square; N. Amer.
informal two-fisted, on the up and up; archaic free-spoken, round.
ANTONYMS evasive, guarded, disingenuous.