You can print on poly using DTG. The biggest issue is finding materials that work.

A lot of polyesters have finishing coatings (weather proofing, uv coating, anti bacterial)on them that make it difficult for the pt and ink to stick.
I have successfully done poly for years but have recently spent more time using the Brother gtx and factory pt and it works well. Also testing different PRETREATMENT too which also worked well.
Temperature and curing is the trick... most users don't bother to print poly for high production because of the time it takes to cure.
Each year each manufacturer slowly figure out ways to better the results (speed up production)

I used the orange hi-vis material to illustrate a lot of issues most people will encounter.
Not all poly fabrics are perfectly smooth and flat.
Any ridge or hump etc needs to be filled with ink to get a sharp flat image.
Otherwise material can show through.

Click image for larger version

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-Credit : Jeff Bailey