Fifty Shades Freed, continues Robbie Collin in The Telegraph, has "zero" nuances, "drab" subplots and a script by Niall Leonard that doesn't "add up".
The Guardian agrees, calling the third film to be based on EL James's erotic fiction "limp and predictable".
Jamie Dornan and Dakota Johnson star in the movie, out in the UK on Friday.
Directed by James Foley, it had its world premiere in France earlier this week.
Fifty Shades Freed sees Johnson's Anastasia Steele and kinky millionaire Christian Grey (Dornan) finally tie the knot.
But their marital life is complicated by her psychotic ex-boss and what Variety's reviewer calls "a thin, illogical abduction climax".
"The series' former tart strain of battle-of-the-sexes comedy has bled almost entirely out of the enterprise," writes Guy Lodge in the trade paper.
"In terms of drama, or melodrama, or just bad drama, Freed rarely delivers the goods," concurs Jordan Mintzer in the Hollywood Reporter.
Pop star Rita Ora plays Dornan's sister in the film and also performs on the soundtrack with former One Direction member Liam Payne.
Writing for Screen Daily, reviewer Ben Croll says the film offers fans of the franchise "all the opulence and tastefully soft-core decadence they've come to expect".
Yet according to The Wrap's Anna Hartley, "the sex scenes feel more like an afterthought, inserted to remind us of the reason the series became such a phenomenon."
The original Fifty Shades of Grey film, directed by Sam Taylor-Johnson and released in 2015, made more than $571m (£411m) worldwide.
Its first sequel, Fifty Shades Darker - scripted by Leonard, the husband of EL James - made $381m (£275m) when it was released the following year.