They might have improved the internal design sure but where does the heat go when you have it resting on a desk? Um.. inside and your desk gets insanely hot then causes a hot plate effect.
If they were both suspended in mid air okay but how is the positioning? If there's better internal heat sink design okay and it dissipates more heat ok? But where does that heat go oh wait right into the surface it's sitting on causing that surface to get even hotter which then negates the fact. If the router was Vertical it would have no choice but to dissipate most of it's heat into the surrounding air.
Asus screwed the design of the RT-AC87R by the dorky location of the USB 3.0 port on the right front edge.
It not only looks messy connecting your HD, but precludes mounting the modem on its side a la RT n 66U so the heat chimney effect would work by dissipating heat up into the atmosphere.
What I do is place the modem on a metal wire rack so the bottom has at least somewhere for the heat to escape.
For sure someone can design a bracket that will allow you to slide in the RT-AC87R, leave the USB port free with some clearance, and allow the modem to sit vertical on its front end freeing up desk space and address the overheating issue at the same time.