You must fight if you want to survive.

Both shows focus on a group of characters given special items (a deck of cards and a VR headset, respectively) by a game master, who is prodding them to kill everyone else to receive a reward. With a lead character whose conscience is too strong to kill anyone.
Alternate worlds are present in both premises. Versus Road having VR and Ryuki having a world on the other side of the mirror, with both of them being the setting for (most of) the fights.

Both are contemporary hero shows, that aired around the same time, focusing on a pair of characters - the Minato brothers in R/B and the "best match" guys in Build.
There are also fourth wall breaking "previously on" segments for both shows.

The biggest difference, at least in my opinion, is that Ultraman R/B has multiple, actually well-written female characters. (And it also does not have a 29-year-old man/19-year-old woman relationship.)

Both shows center around a family of five consisting of an older brother who sacrifices his own interests for his family, a younger brother trying to make something of himself, a younger sister still in high school, and a father a bit on the eccentric side.

The two shows are also rare entrants in their respective series with dual protagonists. Revice having Revi and Vice and R/B consisting of Rosso and Blu.

The heroes in Revice also use inner demons to transform, sort of like how the Ultras choose a human as a host.

Both shows entrust the lead characters with a figure/gun of a blue-haired anime girl that transports them into an alternate reality where they must then fight. But as things progress, things are actually more real than they seem.

Both focus on high school students trapped in their school and forced to fight.

However, Girl Gun Lady has the characters going into an alternate reality in-show, while Kamen Rider Zi-O VS Decade is just full-blown set in a reality that is not the one from the main TV show.

Both are tokusatsu shows written by a well-established female writer about a late-teen/young adult man who is surrounded by characters voiced by famous seiyuu. The lead character lives his daily life with a surrogate parental figure who runs a shop, after losing his parents at a young age.

The secondary, seiyuu-voiced characters also just happen to include an old-fashioned guy, an animal lover who only likes cute things, and a more nerdy/motherly character in both shows. However, while they are both color-coded, the personalities do not align to their similarly-colored counterpart.

And while Kamen Rider Den-O has the characters traveling through time to stop the bad guys, Kikai Sentai Zenkaiger has the parallel worlds come to them instead.

Both Dairanger and Gekiranger are heavily Chinese-inspired seasons of Super Sentai that are also martial arts heavy. Dairanger even has an ambiguously "Chinese" Pink Ranger and Gekiranger's movie is set in China.

They each also only have a single girl on the team (although Gekiranger does at least have other females in support roles) and both have an episode that make use of an actor's real life twin.

Maskman and Ohranger are almost like mirror versions of each other by having a lot of the same elements, but changing its prominence in the show.

They both are military teams that get their power from Ki energy and each member is a master of a different style of martial arts. But while Maskman is very showy with its martial arts and Ki energy, the military aspect is kept at a bare minimum. And while Ohranger very much knows that they are a military sentai, there is also not much gratuitous martial arts outside of actual battles.

Both are family focused seasons of Super Sentai that tend to favor the more eccentric, loud and camp side of the series. While that doesn't mean that they don't also have deep themes buried in there, the two shows are best enjoyed when you're just looking for a good time.

Both shows have Yasuko Kobayashi on the writing staff (she was a sub-writer on GoGoFive and wrote every single episode of both seasons of Amazons), Masashi Taniguchi among the main cast (Go Blue in GoGoFive and Amazon Alpha in Amazons), and put a focus on family dynamics.

Both are sentai that give off a vehicle aesthetic, but are not actually classified as car sentai. Instead they both are about powerful treasures - Boukenger's "precious" and Kiramager's magical "Kiramei stones" respectively - and the team's search to recover them all.

Each also has a Silver Ranger from two different worlds who has personal ties to the villains.

Both are modern day fantasy stories with magic and superheroes.

The two shows each take a similar approach to its villains with demonic creatures taking over the bodies of normal humans after crossing a moral line/falling to despair. Only in Wizard this is not instantaneous and the victim is allowed to fight back or be saved by the hero.

The lead females of both shows also show similarities in their stories with the hero fighting to keep them alive and relationships with her father being examined.