At the current time the best vehicles are only accessable only for their top vehicle squads.
This means that if they want to bring multiple vehicles they would need to fall back to the lvl 2 squads. These are currently still useable but clearly not as good as their dedicated high BR options while only facing BR 4 stuff.
Bringing what is essentially a BR 3 tank or a BR 2 plane just isnt going to cut it in an BR 5 enviroment especially if the US gets their new pershings to sealclub all the sub BR 5 squads.
If the new BR will only add new vehicles to the lvl 3 squads then japan will be at a massive disadvantage due to only beeing able to bring a single viable vehicle into BR 5 while the opposition gets to contantly cycle their best squads contantly as the US has access to top BR squads to sustain it.
If japan has to chose carefully between which vehicle to bring while the US can bring all of them (or even multiples) there is little japan can do agains it and it will be harder to succeed for the entire team.
I know that this is largly a problem created by players cycling vehicles but if only one side is allowed to do that there is little the othere side can do to stop it if both sides know what they do and their skills are at the same level.