That logic isn't really solid tbh. Problem is after 20f, the amount of defense ignore nukes becomes more common, and they are NOT neccesarily a form of "doomsday" nuke, but are just random abilities in that boss' arsenal. Which is worse because you can NOT predict when they will come, they just happen, and result in a dead tenshi, possibly right at the start of a fight!
Furthermore, they are NOT the most threatening thing those bosses may neccesarily have (if your tank isn't tenshi), making the "komachi must therefore be the best" argument based on essentially nothing..
Last, as I mentioned, she's better if you're on a first playthru, or are otherwise above minimumish levels. Because even nukes that do NOT ignore defense that have relatively piercing formulas can 1shot her. When I played for example, even when she was def buffs, some strong bosses (dual laser whatever) nuke for instance would generally own her face. Flying swallow whatever also did the trick, as did a buffed up...fantastic flying object attack or whatever it is that maribel uses on occasion.
Those attacks are generally very dangerous regardless who your main tank is it's true. But I just find that you can generally count on "tanky" characters to at least survive them with high hp, but tenshi has a habit of looking like a tank taking 0s one second, then the next she's flat out DEAD from full hp..from a NON doomsday nuke. It's just a matter of reliability, which is an important aspect for a first slot tank imo.
Plus yeah, there's the issue of her not really being good for her spells. Sword of rapture would be great if it worked ever, but I literally never had that damn spell work with the stats I wanted it to a single time in my entire playthru.