Both Rillaboom and Cinderace are quite strong with their hidden abilities. Grassy Surge, combined with Grassy Glide, gives Rillaboom a very strong priority move. Rillaboom sits comfortably in the OU tier, currently as B-tier in the OU viability rankings. In VGC, Rillaboom's solid bulk and access to fake out, as well as the grassy glide/grassy surge combo, make it a solid choice for both support and offense.. According to Pikalytics Rillaboom is on just over 1/4th of Series 12 teams.
Cinderace is also very strong. Its Libero ability, great coverage and high speed make it a powerhouse, especially in Smogon singles, where it is banned from the OU tier. In VGC, Cinderace has fallen off in usage with the introduction of restricted Pokemon like Kyogre and Groudnon, but in non-restricted formats, Cinderace is an incredibly strong Pokemon that really works well dynamaxed/gmaxed. Since all its moves get the same type attack bonus, it's max moves hit incredibly hard. The only downside to Cinderace is its lack of bulk, but that can be handled with proper play.
If you wanna use Cinderace, I recommend using its hidden ability Libero, an offensive item like Life Orb, and to pick strong physical moves like Pyro Ball, High Jump Kick, Bounce, Gunk Shot, Sucker Punch, U-turn and more.
Inteleon unfortunately has not seen much competitive use yet. It resides in the NU tier in Smogon singles, and has never seen much consistent use in VGC doubles. I did use it once to some success in early VGC 2020 longside Silvally-grass to take advantage of the Water-Grass pledge combo, but it was gimmicky. Even with gmax considered, Inteleon is underwhelming.
I don't play Battle Spot Singles or Smogon doubles, so I don't have much to add for those formats, but from what I know Rillaboom is great in Smogon doubles, and Cinderace is strong in Battle Spot Singles. Hope this helps.
