“Finally, all of you, have unity of mind, sympathy, brotherly love, a tender heart, and a humble mind.” (1 Peter 3:8)
Peter just finished advising employees (or the ancient equivalents) on how to conduct themselves with their employers, and wives and husbands on how to conduct themselves in the marriage relationship, so it’s possible “all of you” here means “all of the persons in the previous specified roles.” In other words, if you’re equally yoked in business*and marriage with a believer, both parties are exhorted to be of one mind equally, to have sympathy and brotherly love for each other, to be tender-hearted towards each other, and to be humble of mind in how they view and treat each other.
This is not a picture of submission-dominance at all. This is a picture of different roles and different functions, but equal power and equal respect for one another, working together, lifting each other up mutually, not one person crushing the other under their feet. Any man who tells you to submit to him while he uses you to wipe his feet is being a hypocrite and in trouble with God. Peter warns husbands in particular in verse seven that husbands who don’t treat their wives with the respect due a co-heir in Christ will find their prayers are hindered. So we know how Christians in authority treat the people that they are called to serve (not to oppressively rule over!) is a really serious matter to God.
Unity of mind likewise is often misused by spiritual abusers, who like to claim it gives them the right to be the group mind and to threaten with hell anyone in their “care” who dares to think differently than the spiritual abuser on any topic under the sun. Instead, consider Peter also requires humility, compassion, love from the leader as well as the follower. True godly unity of mind must require open, mutually respectful discourse between all parties and humbly and prayerfully seeking the mind of Christ together.
*Peter doesn’t specify instructions specifically to employers, but he does instruct all believers to honor all people, love their fellow Christian, and to live as people who are free rather than as slaves. It apparently did not need spelled out further to him that Christian employers should respect their Christian employees as their brothers in Christ and their fellow servants of God. Instead he focuses on the employees of non-Christian, abusive employers, in a day when you were under contract to work for him until he sold your contract. His advice to do good and endure suffering in oppressive circumstances does not give the spiritually abusive the right to mistreat their fellow believers and turn around and demand their fellow believers endure it without complaint.