Board Thread:Boruto Discussions/@comment-33545035-20200220150101/@comment-25122992-20200310173009

For the first part - the control actually matters. It's useful in 1vs1 battle, but as 4-tails Naruto demonstrated, there a great risk of the collateral damage and the direct attack on the allies if they're present. We have yet to know Momoshiki's motives of possessing Boruto, and there's also no guarantee they're benevolent.

For the second part, also connected to the first one - it's not like Momoshiki's possession isn't a risky deal for Boruto as well. Momoshiki himself implied that Boruto is limited by his own chakra capacity even while using Kama. Also, we already saw with Jigen what damage could be done to the body from the overuse of Kama - although he was an imperfect vessel, Boruto is yet to reach the state when he can use Ootsutsuki powers with minimum risks.

Man, Momoshiki even didn't do anything so special. He just demonstrated his innate levitation, boost in taijutsu and a massive Rasengan - something created from Naruto's chakra, similarly to the Rasengan given to Boruto to kill Momoshiki himself. Also, at that point, Boro was just an unstable blob which was easy to kick around, the main problem was only that Team 7 was too exhausted to finish him off.