We are talking about regrounded neutrals here, not mis-wired phases.
Tell me you have never seen a regrounded neutral in a place that has been running trouble free for decades.
Usually these get found when someone adds an AFCI or GFCI

Greg, my error with that UPS system was related to the neutral hook-up ... not the 'hot' wires. I can't claim to remember the details, and I never did understand the issue - only that the APC tech immediately spotted the problem.

Installing several more similar systems since then, the same little detail always makes me pause. The systems have a few more quirks as well, but that's a topic for another thread.

If your generator has outlets on it, the neutral is bonded to the generator frame. Nearly every small generator in the US is designed like this. Permanantly mounted generators can remove this bond, but it's generally not feasible to remove this bond on portable generators as it would float the neutral in the gen when running stand-alone. Leaving the only proper solution as switching the neutral in the transfer switch.

The only installation issue I can see is maybe if the subpanel is flush mounted within the wall, since the transfer panels are usually surface mount. Getting that flexible conduit that comes attached to the little transfer panel into the subpanel may require some cutting of the drywall and drilling a stud. No big deal, but still an added pain.
Are you going to use a hard wire transfer panel model with a WP power inlet mounted outside the building or just a long cord and plug connection hookup?
I’m just envisioning the HO dragging a power cord into the laundry room during a power outage.

The little Hondas are not bonded

The laundry room installation got derailed for exactly the reasons you mentioned. The new plan is to intercept the feeder and put a 125A transfer switch out by the meter. I'll
install a 20A 220V. male twistlock receptacle for the generator input. The meter is in the exterior garage wall
so it will be easy to surface run conduits inside.

Regarding the bonding. I'm planning on leaving the
bond in the main, isolating the solid neutral connection
in the xfer and running a ground wire out for the generator frame to be safe. If a switched neutral is required by code please cite code section.

Is the 20amp twist the largest outlet? If you have a 30 amp I would use that option