not just the tera opportunity cost (which zygarde-complete did not suffer with), terapagos-stellar is a normal-type mon with no recovery in the generation of entry hazards. no useful STAB (though no lack or even need of coverage either), boots vs no boots dilemma, calm mind and rock polish as its only setup options, the transformation foregoes its incredible souped-up multiscale ability for a souped-up cloud nine instead, etc. zygarde-complete had ground STAB that damaged fliers, useful resistances (including stealth rock), dragon dance, glare, and 58 more base HP than terapagos-stellar after transformation (which did need losing some to trigger, but still). of course it might be too much for an OU tier, but i can see why it's being given a shot.