Oofph where to start. When I started I wanted color smart bulbs everywhere. Wish I knew how annoying bulbs connected to switches are.
The latest were some glass break sensors that I have no idea why I bought them, as echo speakers actually do a great job at that.
Ikea and Linkind are 2 companies I love to hate. I've got a ton of buttons from each, at least the Linkind came packaged with an excellent plug for 15 bucks, so can't get too upset there. Ikea on the other hand, I can't stand how many of those buttons don't work, even the button that came with my shade that works perfectly, doesn't work.
Echo speakers... what can I say, I work some midnights and have free time and woot always has sales, lol. I've outfitted 2 other houses with my leftover speakers after I learned whole home audio with 15 echo's just doesn't work, and their is no reason to have one every 10 feet.
Lastly in-wall switches. I bought a bunch of centralites from ebay for cheap to find out they were the companion switch to 3 way switches.
#1 hint I can give is that echo speakers rock, Echo shows suck more than anything has ever sucked in this whole sucky world. Along with that fire tablets make great dashboards, just DO NOT connect them to the internet and update before you know how to root the things to get rid of the Amazon garbage.
Thats about all I can think of right now, though their have been plenty of other incompatibilities, some have been worked out, ie. switchbot bot, those wonderful Tuya 4 gang switches and their 4 button remote, those are real gems for Hubitat.