That would be very bad.
When we had the original extra-tall building rules in place, we had one game in particular back around 2004 where (IIRC) a two hex stone building with six levels (and cellars and rooftops in play) had at least one squad on every level....Japanese and US almost alternating levels. Many had SW's, SMC's, concealment counters, DM and Pin counters. Both hexes had like twelve to fifteen counters in them......back then we used enlarged building printouts to actually place the counters on once things got hairy with stacking, and we just checked LOS on the actual map. Dropping those rules virtually eliminated monster sized stacks, as did cutting back on the number of RG's available in the CG's. Since then I think the tallest stack I have had has been maybe six or seven counters (both sides) in a hex....usually it is just three or four total. Much more manageable......and far less chance of "bumping the table" recon......