if naruto could use BM and SM simultaneously he would be unstoppable ,
sage mode:
- give the user an almost limitless amount of chakra for a set time period , it almost completely upgrades your body , you are almost obliveous to pain and your speed , stamina , strength , power increases massively , on top of this naruto is capable of bending and wielding chakra in more complex advanced ways e.g. throwing a rasenshuriken , however it does limit how many clones naruto can have at one time and unless naruto uses a huge sum of clones as a decoy he cannot go into sage mode while in battle(now that kurama is narutos friend he might be able to merge with ma and pa).

BM:
- this increases narutos chakra amout 1 trilliuon ceptillion bazillion fold , his speed is now unreadable for a sharingan and is almost as fast as using space time jutsu , his stamina has also improved ten fold , his physical form is extended by using kuramas size he can use his chakra to replicate the shape and size of kurama or his paws(you probably dont understand , basically chakra arms and stuff) when he uses this "mode" he can heal almost instantaneously, however this also has a limit on his sum of clones and how long he uses it for , and it is extremely draining for naruto.

naruto is immune to genjutsu now

but if he were to find a way to merge the two:
-untrackable speed possibly even for MS or EMS
-unhurtable(constant healing and upgraded body(i sound like a *** -.-)lol)
-but to top things off sage mode chakra was equal in abundance to half of the users original chakra pool (add the sage chakra and the users chakra is in 3 types of equal amounts).
kakashi has 1 quarter of base narutos chakra and therefore a 6th of SM narutos chakra , and kakashi said that naruto using kuramas chakra has 100x as much chakra , so kakashi has 150th of BSM naruto(if he can do it).
-narutos speed and power and strength would be increased further and has more chakra to go around meaning more clones.
-extended physical form

and lets be honest here, BSM naruto can use more clones , so he could use multiple bijuudama at one time (could be the only way to destroy madaras susanoo).

so do you think if it is even possible that naruto could be unstoppable just by merging the two