I think it's series connection that is usually problematic.

AFAIK the main problem with paralleling speakers with different impedences is that one speaker is usually 8 ohms and the other is usually less, so you end up with <4 ohm effective impedence which only the huge honkin' amps can drive reliably.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8