oo-rah!Dynamax is gone so fake out is viable.
Okay...with the Expansion Pass bringing back Talonflame, along with a whole bunch of other Pokemon, we might need to reevaluate Rillaboom's checks and counters. December might see it drop a tier or two.
Until we know precisely what Bulu gets out of the new generation's selection of TMs (I.E. whether or not it gets Close Combat and especially Play Rough) it's pretty difficult to judge how viable Rillaboom will or will not be once its Hidden Ability is released. Rilla gets Knock Off and U-Turn, which are two phenomenal moves that on their own give it some niches over Bulu as it currently exists.
To add on to you, Bulu has a niche in VGC as it can set up screens, while Rillaboom has access to speed control and can Pivot ( as stated). Fairy is a great type but as of now on Bulu it's only worth a dragon immunity. What will ultimatly set these two apart is their roles on teams and what they specfically use the Terrain for.Rillaboom also notably has 10 more speed, which is pretty valuable especially once Grassy Surge comes out. Yes, it's less bulky (about 2% physically and 7% specially) but it's almost as strong offensively (base 125 vs base 130 Attack), and Drum Beating is a really good move (guaranteed Speed drops are always good pretty much).
I wanted to leave a comment to say Play Rough Tapu Bulu is one of the most overhyped theorymons ever. Despite the Grassy Terrain nerf his Wood Hammer is immensely strong, 2HKOing every offensive Dragon with Band, actually OHKOing Latios after Stealth Rock. If you want to tank it you have to use far beefier resists(mostly Steel types but also some Poison, Fire and Grass types), possibly *4 resists like Corviknight (although some bulky *2 resists like Melmetal work). Considering Hammer, Superpower and Stone Edge are effectively mandatory, Horn Leech tends to be more useful than Play Rough on a Band set since Bulu wants a safer option that still hits very hard and greatly extends its durability, while other sets like SD, AV, even fringe stuff like Subseed are very strapped for moveslots and don't really have space to fit Play Rough in. I am not saying it will be a completely useless option, however it will take far, far more than that to make choiced and offensive SD sets more attractive over U-turn and Knock Off (two moves that have like a million useful applications) and the higher speed that allows Rillaboom to comfortably outspeed Heatran.Until we know precisely what Bulu gets out of the new generation's selection of TMs (I.E. whether or not it gets Close Combat and especially Play Rough) it's pretty difficult to judge how viable Rillaboom will or will not be once its Hidden Ability is released. Rilla gets Knock Off and U-Turn, which are two phenomenal moves that on their own give it some niches over Bulu as it currently exists.
All legendaries other than Zacian, Zamazenta and Eternatus can Dynamax, this has already been tested with the ones that are hidden in the current version of the game like Mewtwo. It's worth noting that Rillaboom's G-Max move will allow it to avoid redudancy with Grassy Surge, although we don't know its effect yet. Even then, Max Overgrowth can be useful to come on top in terrain wars so Rillaboom can still retain that option too.And for those looking here for VGC advise, as of now it is unconfirmed if other Legendaries can Dynamax outside of Urshifu, so it could be possible Rillabooms Dmax/Gmax form will outclass Bulu on everything bar setting screens. (although this is pure speculation)