Well I don't design them, I do the welding and heavy current soldering (radio guy here, this a power dude's design, or at least someone is his department)

One potential (pun intended, kinda) problem is that slight differences in characteristics between individual BMS boards could lead to strange things happening - if there is even a few mV difference between individual board calibration, placing several units in parallel could lead to things getting out of hand. This is especially true if the BMS has a lot of proactive monitoring on discharge (simple boards don't monitor the discarge so much, if at all - the cheapest are more corerctly called charge controllers)

Now, in a perfect world, good quality compnents, good design, attention to grounding to prevent EMC issues, low RF environment, stable temperatures, good quality cells all similar in characteristics, yeah, you MAY get away with such a multi-BMS pack. But I'm not sure I've seen that design commercially except maybe in EVs. So if you try it, be careful.

I have no idea if any given BMS chip would even tolerate that (check for reference designs on datasheets) and beware that most Chinese BMS boards sold are adequate-but-shoddy in my limited experience with them (solely from fixing up my friends power drills or converting old NiCad ones to lithium)

Like i say, I don't design them, I just spend my time on quiet shifts doing the tabs (bigass tab welder) and 0AWG soldering (gas torch) because I have years of experience with both procedures, But mainly because when they tried getting their usual contractors to do that kind of work they had several serious fires and two explosions that cost a lot of downtime and a lawsuit. So it was "hey old guy - can you help us with these?"

Batteries are dense in stored energy. Big ones are scary. Huge ones can kill. In many ways.