Can anyone give an idea about out of the box thermals for the NUC7PJYH1 (J5005)? Also if the fan fails how easily accessible is it to replace? Can you just take off the top and remove the fan or do you need to get to it from the underside which is considerably more work? Thanks.
Removing the top cover will not give any access to any part in the NUC. You can remove it and replace with other cover if you want. If you want get access to the thermal solution and/or CMOS battery, you indeed need to remove the board. In order to remove the board, you need to remove the bottom cover, disconnect some cables (like antennas cables from WiFi module, digital microphone array), unscrew to screws. See this video (this is for NUC7i5BNH, however, it is similar in your NUC) https://www.youtube.com/watch?v=Tn5cM0c7LpQ
First of all, it is a blower, not a fan. Blowers can be more difficult to find if you need to replace them (though many used in the NUCs have become available via Amazon and elsewhere).
I personally consider the thermals of this NUC to be somewhat problematic. The Atom, Pentium and Celeron processors used in the NUCs do not support the PECI bus, so fan speed controllers -- such as that included in the Super I/O devices on most motherboards -- do not have access to the actual processor temperature. In these NUCs, the default fan speed control settings in the BIOS use secondary temperature sensors to determine whether increases in fan speed response are necessary. This works fine most of the time, but sustained high processor activity can result in processor throttling.
Sorry to be the bearer of bad news, but this is reality. The NUCs with Core i3/i5/i7 processors do not have this issue; the PECI bus is supported by these processors.