- MicroUSB for power - limits you to 2A, very bad
- Crap/slow eMMC - Look at random I/O as main method of judgment
- USB Hubs where they shouldn't be needed - OrangePi+ 2 made this mistake, get OrangePi+ 2e instead
- Poor board thermals - a good SBC will have a copper layer below the SOC to spread the heat across the PCB
- Non-mainline support - It might be great hardware, but its a dud without software
- Bad Power Supply - A phone charger is not a SBC power supply
- Slow MicroSD - Buy Samsung Evo 32GB cards, test immediately. Even with a good brand like these, 2 out of every 5 won't even do 10MB/s of write, or 2MB/s of random I/O, making them not a class 10 card. Exchange the bad ones, buy used if your on a budget.
If that's the only thing wrong you can buy a few chip heat sinks and set it in a thermal epoxy to turn the entire thing into a thermally massive block. Even crap air flow should clean that up and then you don't need a case. You can also thermal glue it to a large bit of metal if you have a metal desk or something
Other bad moves are dumping 1.3v or 1.4v into the SOC and not allowing the user to adjust that voltage, cause then the board is just generating heat needlessly instead of running at 1.1v or so.
Aren't there a lot of counterfeits that's why?
That being said, Samsung isn't performance testing each microsd card, so you need to bench them ASAP when you get them to make sure your getting above 2MB/s in random I/O and around 20MB/s in write speed. If they fail that test (which if your buying more than one, you'll run across), just get it exchanged with the seller, no reason to hang onto a poorly performing card.