Key features

• Arcade-ish driving,
  
• They improved physics (since Steel Titans),
  
• You can do Backflips!

Just Better
I played both MJ Steel Titans and both times I was loudly cursing its physics. It was bad. Stunts were pain to perform and racing was a massive struggle. Showdown somehow fixed that all.
Trucks no longer soar on a flat surface. You can easily initiate a stunt and continue performing it. You can actually feel how you are getting better at it, and so it becomes fun, as it was intended.

Moreover, the graphics is visually better, it's less plastic. And runs great with older hardware (system specs are below), except for one map, I'll explain further.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3704765792
Just Events
Showdown doesn't have Free Roam World with secrets, like Steel Titans did. It focuses on events.
Racing, stunts pvp, head to head in a very tight paths. That's all. Simple, but the amount of them will take about 20hrs (my case) to complete. They take place in four different areas: desert, autumn forest, icy plateau and sunny jungle/volcanic island.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3705187437
There, in Hawaii, to my big surprise, one sunny map tanked my frames - it was the only one map that drastically worsened the performance, didn't occur with any other ones. Hmmm 🤔

Back to Stunts!
I really need to expand a bit. You can ride sideways, ride front wheels, rear wheels and either forward or reverse - count as different stunts! Flip you truck sideways, multiple times, drift, do donuts. And, naturally, build a massive winning combo. That mode is very good. It was unbelievably annoying in Steel Titans, but here - it's a blast!
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3706605857
Hidden Challenges
Each truck has an alternative skin - livery. To unlock them you either must complete related challenges, or just win certain events while driving a certain truck. Once that's done - equip a new, different colour top and back to the racing you go!

Monster Jam™ Showdown is a nice game. Got my copy during sales and quite happy with its discount price.
It's no way a realistic auto-sim, but more of a chaotic racing game. Not exactly off-road as it claims to be, as each track is a sterile straight line road, varying in wideness, but close to it.
Worth trying though!