Where and how often are you traveling, as this can make a big difference? As can the particular airline policies.
In general though, these cases sound too big. You’re just asking for trouble (i.e. a destroyed, delicate modular synth) if you don’t plan ahead.
I took a 12U folding Amalgamod case (about 10 kg fully loaded) on various European budget airlines, and while it was OK in the end, there were a few ridiculously over booked flights where I could’ve ended up having to check the case in (to the hold) which would’ve been a disaster.
I personally - if paying myself - won’t take much more than a smallish 3U or 6U I can almost guarantee to carry on.
Others here have good experience and will chime in.