I have some neigbors which doesn't have zero. For me it looks like they are somehow broken. Is that true or is it okay if some neighbors have invalid transactions?
For every invalid transaction from neighbor you will see one of those logs :
"Transaction processing runtime exception "
My interpretation : received data is corrupted in some way. Should be rare, unless your neighbor is voluntary spamming with garbage.
"Error accessing persistence store."
My interpretation : Problem maybe on your side. Your IRI is unable to persist the transaction received from your neighbor in local DB.
"Received an Invalid TransactionViewModel. Dropping it..."
My interpretation : your neighbor send you something that looks like a transaction, but it is not valid. I guess that your neighbor and you aren't running the same version of the IRI.