Lone Wolf is the bomb. Wish there were other starts where you have a "PC". Makes the game feel even more "personal"
Hopefully if there's a BB2 this will just be a standard feature. Always felt odd to me to be cast as this invisible management character who leads the company but doesn't actually fight. And from a gameplay standpoint it's not uncommon in SRPGs for there to be character(s) that cause a fail state if they die, i.e. Marth in Fire Emblem.
Bonus points if you're allowed to choose a background, stats, etc that cause some unique events and interactions throughout your playthrough.
You need to implement it carefully. On the balance I agree with you, but without safeguards (I haven't played the new DLC yet, but I understand that Lone Wolves start with decent armour at least?) you're going to have a character with whom, as the game progresses, there's an increasing need to treat with kid gloves. And the fantasy for most people with their main character isn't to be babysat, it's to be a powerhouse. So that might deter them from making it the standard rather than an option.
I think a good compromise solution would be this: rather than making them a regular bro, give them a specialised role of their own that feels fun and powerful without putting the player at risk of sudden death. Something like the heroes of Heroes of Might and Magic II-III, hanging back but casting devastating spells (which wouldn't be appropriate for Battle Brothers, of course). Arming them with something like a ballista or a cannon seems a setting-appropriate alternative as well as providing another avenue for upgrades, and there's no reason the enemy couldn't have someone in that same role as well. You wouldn't expect your average group of bandit thugs firing siege weaponry at you, certainly, but a company from a noble house could easily have an officer at the back manning the company cannon.
It varies depending on the length of the game, of course. If you can get through your average Battle Brothers campaign in twenty hours or so, it's not a huge deal if you risk losing your progress to an unlucky headshot or two on your player character. But for people who rack up huge amounts of hours on a single campaign, or if it's a longer game, some people just won't be able to deal with that level of ever-present risk by having a player character on the battlefield.
That said I've been playing with a self-appointed player character (where if he dies, I lose) since the game first came out, so maybe I'm making too many assumptions and a lot of average players would really go for it. It's just that generally, in less
communities, there's so much talk about savescumming and easy mods and whining that they generally come off as a bunch of total scrubs who'd throw a shitfit the first time an orc berserker beheaded their darling avatar.