Radio Times enjoyed the story calling the script beautifully written. "What’s often rewarding about Doctor Who is that – beyond rewrites, budget constraints, casting and performance – it allows an authorial voice to sing through. It happened for Munro in 1989’s Survival and does so again in The Eaters of Light. It’s a beautifully written script that feels at one with half-remembered Celtic myths."
Digital Spy agreed the story had a both a strong sense of place and a formidable grasp on character. "Some fantastic location work – many evocative shots of a moody, misty Scotland – helps bring Munro's script to life as she milks the 2nd-century setting for its worth. She finds both humour and pathos in the past, such as the Doctor's using a totally anachronistic bag of popcorn to frighten the natives, followed by Bill's horror at the shockingly low age of soldiers on the battlefront."
The Telegraph also appreciated the Direction. "Director Charles Palmer made full use of the sweeping Highland landscape, while the script had fun with the Scots theme, with references to the permanently damp weather and a fatal absence of sunlight being “death by Scotland”. By the end of the episode, Capaldi even wanted to stay."
The Mirror felt the story was safe filler material with some niggles. "The Light Eaters are visually glorious and using them sparingly helps build the tension for their screen appearances. This series of Doctor Who continues its tightrope act of exactly how gory can corpses get before the watershed and this is pretty gruesome. The bodies of the fallen soldiers and villagers are truly unpleasant, but more than enough to punctuate the danger for anyone who crosses the Eaters' path."
AV Club praised the script written by veteran Doctor Who writer Rona Munro "She skillfully mixes the political and the personal here: The Roman army is a weapon premised solely on overwhelming force, one that relies on taking hundreds of scared teenagers and siccing them on a bunch of peaceful farmers who are in the way. The empire is terrible and vast, but only in aggregate, and that reality makes the cowardice of the surviving soldiers all but inevitable."
Ars Technica felt the story was a good introduction to the series "There's a portal that has trapped a hungry beast between dimensions; the Doctor jigs his way through problem-solving while reminding Nardole and the audience that he's an old hand at this kind of thing and a classic Who story device features, sidekick Bill is separated from her time-travelling pals for much of the episode—leaving her to untangle yet more of the Time Lord's powers, such as the telepathic link from the TARDIS that auto-translates any language to English."
However Games Radar felt the threat in the story was confusing "Unfortunately the monsters are pretty badly explained; we know they eat light, but somehow being exposed to light in great amounts is their greatest weakness. They serve no other purpose than to be the baddies of the episode, and are (as usual) billed as a threat to the entire universe. No nuance, just lots of teeth and Medusa-y tentacles"
Den of Geek also felt the episode was missing something. "What The Eaters Of Light lacked for me was a sense of threat, a strong monster or force to push against. The creature we got was an impressive looking beast for the most part, the one who keeps popping through a portal when able and allowed to wreak havoc. But whereas there are moments in this run of Who that have really dug under the skin and been quite creepy, this time it felt like we got a decent enough creature, yet the sense of peril didn’t come across for me."
IGN praised the supporting cast "The supporting players of Romans and Scots are all pretty good and an improvement over many of the guest stars from earlier this year. Bill’s interplay with Lucius (Brian Vernel, who Star Wars fans might recognize from The Force Awakens) regarding his romantic intentions is pretty funny, and the pain of the girl Kar (Rebecca Benson), who has lost everything, rings true."
The Reel Bits felt the episode was old school Doctor Who "The Eaters of Light’ is a solid if not outstanding historical adventure. Spinning its wheels slightly, it’s reminiscent of the show’s original ethos of being an educational outing for kids. Indeed, if the special effects and outfits hadn’t been updated, a episode that primarily hangs around in a handful of locations with Roman soldiers in stock BBC costumes would fit right in with the original series."
Finally TV Fanatic felt it was a solid episode. "There was plenty to enjoy here, from Nardole "blending in" with the natives to the Doctor's speech about crows being in a huff to Bill's realization about the TARDIS's translation feature. Though once the Doctor volunteered to guard the gate, every likely viewer knew that someone else would stand up and do the job in his stead. Seriously, there are two more episodes left in this season after this one!"