I'll cut to the chase: Scoob! is a boring and unfocused mess, too busy selling toys and setting up sequels to bother with being entertaining.
The new film, premiering in people's homes because movie theaters are closed, is an attempt to launch a new shared cinematic universe based on old Hanna-Barbera cartoons. The result is characters from other old shows getting awkardly interpolated in the story of the Mystery Inc gang. Dynomutt and the Blue Falcon (Dynomutt, Dog Wonder) show up, as does DeeDee (Captain Caveman and the Teen Angels, whatever that is). The main villain is Dick Dastardly (Wacky Races), who comes complete with some Minions ripoff robot sidekicks and a plan that involves opening a portal to the underworld or something. It's all very incongruous with the classic Scooby formula, in which the supernatural was almost always revealed to be smoke and mirrors.
The humor is aggressively topical, referencing smartphones and Tinder and Hemsworths. None of it is very funny, least of all a cameo by Simon Cowell. Is he still relevant?
Like the recent Charlie's Angles reboot, Scoob! drags its source material into a new era by discarding its original genre. It's no longer a mystery, but a sci-fi/superhero/action movie. The franchise has been so thoroughly put through the generic blockbuster machine, it has lost almost all of its charm.
The hallmarks of the franchise (chase sequences, unmasking the bad guy, the "meddling kids" line) are there, but they feel clunkily tacked-on, they don't feel organic in this story of world domination and portals to the underworld. See 1998's superb direct-to-video film Scooby Doo on Zombie Island for an example of how to turn a classic formula on its head in ways that are clever and interesting.
The hiring of a celebrity voice cast was another mistake. Will Forte, Amanda Seyfried, Gina Rodriguez, Zac Efron, Ken Jeuong, and Mark Whalberg are all fine, but they do not really add anything. Frank Welker returns to voice Scooby (as he's done since 2002), but to replace him with Efron as the voice of Fred, the character he's been voicing since 1969 seems like a huge insult. It's like when The Jetsons movie version from 1990 dumped the OG Judy, Janet Waldo, after she had already recorded her lines for a more relevant celebrity voice (the relevant celebrity in question: Tiffany). The current voice cast from the Scooby cartoons deserved to be in this film.
Of all the updates given to the franchise, curiously the same is the dumb old Mystery Machine. If I rebooting Scooby Doo, that would be the first thing to go. And how come Fred got to ditch the ascot, but Daphne still can't wear pants? Either keep the retro outfits, or update everyone's look.
Not that Hanna-Barbera was ever known for its high-quality animation, but if Warner Bros, really wants to turn these old shows into big-screen franchises, they will have to do better than the chintzy animation of Scoob! The backgrounds have close to zero definition, everyone's hair looks weird, it's all just cheap-looking. Courtesy of Reel FX Animation, the visuals are nowhere near what Disney and Pixar are doing. If the studio wants audiences to pay movie-theatre prices, then it has to look better than a Saturday morning cartoon.
Speaking of the animation, why does so little of it recall the iconic Hanna-Barbera aesthetic? Aside the from the design of Scooby and Shaggy, it doesn't really look like Scooby Doo. Nowhere is this more evident than in the dutiful recreation of the title sequence from Scooby Doo, Where Are You? They should have updated the visuals, for sure, but paid homage to the original. The Peanuts movie from a few years ago did a good job of translating cheap cartoon style into computer animation while retaining the same essential style.
Scooby Doo is a 50 year old franchise that has seen reboot after reboot. There are have been many versions that are better than Scoob! and even some that are worse than Scoob! Go watch the original series, go watch the live-action movie with Sarah Michelle Gellar, go watch the one where Shaggy becomes a werewolf, go watch the one where they solve a mystery with Sandy Duncan. There are a lot of interesting directions this franchise has gone in. Scoob! is not one of them.