| Dakitten |
You are a saint and a scholar, and I've quite enjoyed your other bits of work on polymorph! I actually rather like this version of the Envoy, so I'll share my thoughts, concepts, and encounters while playing one!
1. Augments are technically equipment, and an argument could be made that polymorph and augment interactions are no good... but you aren't technically using polymorph, you're using UMA shape change with polymorph-compatable forms! While the question of size comes into play with this (maybe wanna stick to medium or small size aasimar/tiefling envoys as a result), this has two potential views in tandem with subtype shenanigans with powered armor as far as I can see...
A. Since it is another permanent shape you can morph into, you can purchase augments unique to each shape. This is how we play it at my table presently, and although it didn't really mean much at low levels save for rethinking my $ investments. In the teens it has become a pretty nice method of using cheap augments (I <3 you, species grafts) to gain even greater versatility than could be handled with expertise.
B. More fuel for the argument with your GM to let you keep your augments because LFAN is amazing and I don't like giving it up ever.
2. Being free of requiring an expertise focus does free up any expertise talents that don't require expertise dice. Examples that jump out to me include Universal Diplomat, Biometric Implants (very fitting), and Surgeon.
3. You only need so many forms to be effective. Infiltration aside (make a custom super-human and maybe a custom super kobold or other reptilian to get around anywhere unquestioned), you only need a pinch of the weird to get the most bang for your buck. Pester your GM for unique forms and unique form abilities, such as a Yrgytchees' gravity manipulation or a Stomakera's hide in plain sight and stealth boost. Make sure your mesh generics are support-focused as hell, and ideally scale more uses with level too. The Perfect Storm adventure path is AMAZING for this!
4. This is the big takeaway, and why I don't understand the hatred. This is essentially an any time permanent scaling powerful SLA that only requires you sacrifice Expertise and Expertise Talents... but you can polymorph your way into almost equal bonuses to niche skills ON TOP OF being a different being on the fly! In the beginning, it is going to sting hard and out loud for everyone to deal with, but by level 20 you can turn into a Voidshard Vampire and create an endless army of thralls (You're a shapechanger, so you're carte blanche ALWAYS the vampire that sired them even after changing to another form!) before turning into a Lightweaver and perma-invising and then shifting into a Solmyr and using your RP to force good saves to be rerolled en masse, and then turn into a Mummified Prophet and solar battery up another 4-5 RP. This is a complicated, low up front reward option for the Envoy, but in both flavor and long term power it is an amazing investment completely different from any other option in the game.
5. This is a small bit but a goodie... powered armor + onboard computer + spell gem understanding improv + spell chips + polymorphing into another type/subtype = A very big surprise when your otherwise normal human looking infiltrator shape starts unloading and breaking the action economy. You're a shapechanger, so there's no true sight magic detection that will catch you smuggling in murder tools, and dear gods this leads to some AMAZING espionage.