So the 2015+ Sienna has inferior AWD?
Toyota changed the AWD system on the Highlander and Sienna to a much simpler "PTO" (power take off) style unit - far fewer mechanical bits, meaning lighter, easier to service, and so on. But, it's no longer full time 50:50 - it's more akin to the older Subaru system found on their automatic transmission cars from the 2000s where it would operate with only minimal power to the rear wheels during light throttle straight driving conditions - engaging more power rearward as needed.
It's a very good on-road system, but doesn't have the same AWD "grunt" that the older 50:50 system does. For 99.99999% of drivers, it's excellent. I just prefer the feel of the older system. I also do take the HL off road periodically, so I like the older system. Sienna owners? Uh, no.