Yes, you MUST keep the cells balanced. This simply means keeping the cell voltages equal.
You are confusing balancing, which is (theoretically) only for maximizing energy storage capability of the aging pack, with cell-level monitoring, which is (again theoretically) for safety and protection. I could consider accepting the answer that the latter is a "must", but the former is clearly not.
Talking about "keeping the cell voltages equal" is nonsense. What is equal, anyway? Within 0.0001V? Within 0.1V? Even in a perfectly balanced pack, voltages will be significantly different over the operating range. For example, if the pack is balanced at 100% SoC as usual, then there are going to be significant voltage differences (>0.1V) at, say, 5% SoC, under considerable (over 0.5C, for example) load.
If this balancing is a must, why it's not universally used? Why most power tools don't do it?
That's about balancing. Then, even the cell-level monitoring is not actually a hard requirement; definitely not a generic "must". Companies such as BOSCH do not do it on many of their 6s and smaller li-ion packs.
Though, for such big cells (or paralleled cell groups - doesn't matter as long as the parallel connection is securely fixed during manufacture of the pack), cell-level monitoring could be important, but even more than the cell size, it depends on the number of cells connected in series. The more you have cells in series, the less relative contribution to the total voltage from any single cell. For example, almost no one uses cell-level monitoring on 2s packs, for which there seems to be total 100% industry expert agreement that it can be considered as a single 7.2V cell and managed as such. Above 2s, the viewpoints start to spread.
Now, you had one thing right - designing a BMS is not easy. It feels like an easy task, but the devil is in the implementation details. This can be easily seen by analyzing existing BMS's. Every single one I have seen has been faulty by design, and possibly dangerous. The most typical issues are: 1) high quiescent currents, deep discharging and killing the cells in typical use cases that the designer still didn't anticipate, 2) lack of
really necessary protections due to the limited lithium ion chemistry knowledge of the designers; instead, all kinds of fancy "we-think-we-need-this" features are implemented, 3) stuck on balancing resistors, either just killing the cell, or worse, when combined with not-designed-for-worst-case thermal misdesign, heating up the adjacent cells over the thermal runaway onset temperature (about 150 degC for LCO).
I have designed my own, as well, and sold it in fairly small numbers for suitable clients for specific purposes, and I'm quite darn sure I still have some bugs there, as well
. In any case, the correct order to design a BMS is:
1) Make sure it doesn't heat up or catch fire in any possible use or misuse scenario - do the thermal analysis assuming it's coupled with the cell, then thermally insulated for little cooling effect
2) Make sure you have correctly learned and identified all critical safety parameters of the cell chemistry. Unfortunately, forum posts and easy-to-digest fake information sites such as Battery University are not enough, you need to attend some courses or read enough research material on actual cell chemistry.
3) Implement monitoring on these critical safety parameters and react by shutting down any operations if these are exceeded. Make sure you have this completely working before implementing anything else
4) Implement non-critical "comfort" features such as balancing. Make sure these do not prevent point 3) from working.
Last but not least, modern li-ion cells from the brand manufacturers are very well protected on chemical and physical level. This is necessary, since the industry experience has shown that the BMS's do not protect reliably due to so much misdesign and misuse; and even if correctly designed, there are limits on what they can do.