All actual Christian denominations are significantly deficient; because schism is an evil.
(Although of course not necessarily the worst evil; nonetheless schism is always an evil - always destructive of The Good to some significant extent.)
After schism, all parties are weakened, those who stay behind, and those who go.
Those who provoke schism (and stay behind) may be doing an evil, those who go into schism (and leave) may be doing an evil - and the necessity for schism (when it is necessary) is itself an evil, the product of evil of one sort or another.
A world full of schismatic churches - this world - is seriously sub-optimal.
When a church divides, what remains is less than the whole - the process of division is destructive.
Even the best surgeon cannot cannot divide flesh without destroying tissue in the process; yet surgery may nonetheless be necessary.
Even if we accept that the Eastern Orthodox Church is the remnant of the first unified catholic Church, it is but a remnant; Orthodoxy is a lesser thing in a world where Christainity is divided - the remnant must organize in response to the schism and this does harm.
Precisely the same applies if the Roman Catholic Church is seen as the remnant of the first unified catholic Church.
But Christianity has been through many, many schisms - therefore the Christian world is by now much diminished.
Consequently, as things are - two thousand years down the line of schism upon schism - no Christian denomination can legitimately claim wholeness, none can claim the exclusive path; yet denominations which are eclectic, non-exclusive, 'ecoumenical' (in the modern sense) are merely apostate, weak, approaching spiritual death.
This is why schism is bad. Schism makes a difference, a permanent difference, an unrepairable difference.
What is done is done and cannot be undone (although it can be repented and forgiven).
(This is one reason why the Christian sees the world as finite; history is an accumulation of the effects of sin. The soul can be saved, but the world cannot.)
At this point in things, the damage to the world precludes certain options which were possible in the past.
One of these impossible things is finding the true Church here on earth in institutional form and submitting to its teachings, holding nothing back.
Evil has learned much over the millennia, trail and error has yielded subtle strategies which easily turn the already weak and corrupt human will away from salvation and into the service of evil.
Read the The Last Battle by C.S Lewis to see how it works.