The CanSino vaccine also uses Ad5 as it's viral vector. An immunity to Ad5 from say Sputnik V would apparently prevent you from being able to get the CanSino one should Sputnik V prove ineffective for you. You'd also not be able to get a booster of Sputnik V later.
There's lots of adenovirus strains and a few other vaccines are using them for their delivery vectors. Ad5 is an edited adenovirus that has it's replication system retarded/removed. There's other articles I've seen but can't find now lamenting the use of Ad5 specifically. There was also the STEP study that showed increased HIV susceptibility in people that had received Ad5 vaccines.
I am not a medical professional but I do understand immunology is extremely complex. Effective vaccines in vitro aren't necessarily effective in vivo. There's also complex issues around dosing which is part of human trials. Even if a vaccine is "effective" it's not necessarily obvious or known what sort of dosing is appropriate for wide deployment in the general population. A completely "effective" vaccine at the wrong dose can end up being ineffective.