The archdevil is a brawler. A big effect at a single point. We are, presumably, fighting with armies here. The archdevil could be fantastic if it happens to be at the right place at the right time, but otherwise it is just a single threat that can be focused down or avoided. There is a reason why generals don't lead the charge. One guy an their elite forces with the best equipment and support can make a massive impact on a battlefield, but ultimately it is a risk. Even the best giant monster with a sword doesn't do so well when they look up and see dozens of enchanted ballista staring at them...
The lich, on the other hand, is a strategist. It plonks down free units every turn, which presumably can be revised if need be(as can the archdevil's gear once their sword can't cut through the latest magical energy shields). Extra units means a larger front, so less chance of being flanked and more chance of flanking the enemy. More troops means more reserves so you can cover your forces as they regroup.
The lich does unknown curses. These may be large-area morale drops, like an incursion of irritating insects. They might be a single-target thing, like immaterial shackles slowing down the enemy champion. They might be logistical issues, like toxic ex-rodents who like to go swimming in the other team's water supply. Or perhaps a localised effect, like a blinding spell that hits a dozen dudes... It is unlikely to be all of these things but any one of them could turn the tide of battle. Most fights end because one side breaks. making the other team want to be here less than you has decided many battles. If it can take away the devil's mobility, or reduce it from an unstoppable force to just being tougher than anything else then the likes of the devil lose their ability to be committed into the enemy lines. They are still a strongpoint that is easier to go around than confront, but it is possible to go around them. A sudden incapacitation at a single point in the line can be devastating and forms the basis of much of antiquity's tactical doctrine. Now while this is pretty much exactly what the archdevil is, the archdevil is big and very very obvious what with being surrounded by screaming loons, while the lich is a back-line force so if it can incapacitate a force it can likely do it at a time and place of its choosing with very little warning.
The archdevil is a single point of power, but also commitment. If the enemy has a single weakness, and the devil exploits it, then it can win a battle. Break the vanguard, kill the commander, harass the auxiliaries, these things can be done with a single unopposable force. But it bleeds our armies of reserves and is prone to being surrounded as our nearby forces blindly rush into any battle they can find. It is the champion for legendary victories.
The Lich is control over the battle. Creating and exploiting multiple weaknesses. It is delaying and tiring the enemy's vanguard with undead while weakening their reserves as their unsupported troops break and run. It is weakening the enemy forces who breach our lines while the reserves buy time for the broken forces to recover. It is sapping the enemy will so that a route turns into a withdrawal. It is the champion for consistently good performance.
If we want a hero who challenges the enemy champion to a duel and rips them in half with its bare hands, then the horror is the clear choice. If we want a champion that laughs at being challenged, turns their champion's legs numb, and then orders an artillery barrage onto that position while expendable undead keep them engaged, then we want the Lich.