The number one issue in regards to OSAS is it redefines salvation, by necessity, to being something that is abstract or positional.
It is impossible to hold to OSAS and teach that salvation has anything to do with manifest purity of heart and a true setting free from sin. That is the major issue.
The issue of "failure" can only be related to sins of ignorance. Willful rebellion is the forsaking of the light and the very moment one does such a thing spiritual death is wrought. To deny that is to believe that one can sin and not surely die.
If one has an understanding of the right way and willfully refuses and instead goes the wrong way, it is an act of death. It is the manifest state of "doing evil" that God saves us from. God simply cannot and will not forgive any individual who is in a state of rebellion. OSAS ignores this.
OSAS proponents will NEVER EVER EVER quote scriptures like these...
Isa 55:7 Let the wicked forsake his way, and the unrighteous man his thoughts: and let him return unto the LORD, and he will have mercy upon him; and to our God, for he will abundantly pardon.
Pro 28:13 He that covereth his sins shall not prosper: but whoso confesseth and forsaketh them shall have mercy.
Jas 1:21 Wherefore lay apart all filthiness and superfluity of naughtiness, and receive with meekness the engrafted word, which is able to save your souls.
Jas 1:22 But be ye doers of the word, and not hearers only, deceiving your own selves.
The engrafted word is what actually saves the soul. The Word of God has to be received inwardly in order for the salvation of the soul to take place. This cannot occur in rebellion which is why James teach that we are to lay aside ALL filthiness and superfluity of naughtiness, not just some of it.
Repentance purges the soul of rebellion where we may receive God within. Look at the words of Jesus...
Joh 6:63
It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.
Now compare to what Paul wrote...
Eph 2:5 Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved
Eph 2:6 And hath raised us up together, and made us sit together in heavenly places in Christ Jesus:
OSAS teaches that salvation is purely positional in nature. It utterly denies the union with God that is manifest in the real salvation experience. When one is in union with God one cannot be in rebellion. The moment rebellion takes place is the moment there is no union. Salvation is FROM sin, not in it.
OSAS teaches saved IN sin.
Statements like this set alarm bells off for me...
If you mean "willful sin" in that statement then you do not understand salvation. If you mean sins of ignorance, ie. doing the wrong thing unintentionally then that is a different matter.
Willful sin is always a sin unto death. Always.
Satan wants people to believe that willful sin is not unto death. If Satan can convince people that they can engage in a single act of rebellion to God and not surely die then those convinced are doomed. The one lie of "ye can sin and not surely die" is the most dangerous lie to ever be mouthed into existence. So many believe it even though it is the very first lie of Satan in the Bible.
Be very, very careful in what you believe and examine everything very carefully. If your heart is genuinely honest you will know these things are true as none of this is complicated.