I have gigabit internet and Ethernet drops under each tv but $500+ for 3 Apple TV 4K or shields are out.
So that leaves the fire tv cube, mi box, and the Apple TV 4. All 3 wired for 100 mbps. Which platform will work best with a harmony remote? Will support numbers, dvr, guide buttons etc
If price is a concern, your best bet is Fire Stick 4K. You can add a USB ethernet adapter and USB IR receiver (necessary for the remote, unless you're planning to use Harmony Hubs) for another $20 or so per TV.
The FireTV cube and mibox are both terrible at video decoding.
The 4K stick with Ethernet adapter is your best bet. There should be a new shield coming out soon too, so the old ones may go on sale.
FYI, All I use for clients are three FireTV Cubes and I haven’t had any issues whatsoever.
What would I be seeing as far as issues with them, if I did?
Of those three options, I agree with the others in saying the Fire TV 4K stick is your best bet.
However, I would encourage you to look to eBay for Apple TV 4Ks. You can regularly find them new/sealed for around $140. With a coupon I bought one last month for $116 (about $122 after tax).
Amazon Warehouse Deals has plenty of listings for open-box Apple TV 4K around $132 (albeit in "acceptable" condition): https://www.amazon.com/gp/offer-listing/B075NCMLYL/ref=dp_olp_all_mbc?ie=UTF8&condition=all
(Of course, you can also save a few bucks on a Fire Stick 4K by buying it refurbished for around $45: https://www.amazon.com/gp/product/B07F84C1BL/ )
I personally find Apple Devices overpriced and too limited on what you can do with them.I had ordered an AppleTV 4k had problems with HDR and SDR on my TV ... also I could not install the Apps I like. I sent it right back. The ShieldTV gives you more bang for the buck.