No, not problematic if as you say, they provide security and stability fixes. That's a big IF.
Not problematic if the older kernel has the drivers and interfaces you need. Another big IF.
My experience with Freescale hasn't been so great. The chips are fine. Wonderful even.
About 6 years ago, I was working for Jacques Electronics, doing embedded work. They were using KaRo TX27 modules, based on the iMX.27 SoC. Latest kernel at the time was 2.6.33, but KaRo were shipping a slightly patched Freescale kernel: release 2.6.28, which was at the time at least a year old.
The older kernel lacked support for the I²S multiplexer present on the iMX.27. I wound up backporting ALSA from kernel 2.6.33. (There was a lot of video stuff they were using that simply wasn't ported to the newer kernel.)
Then more recently, different company, now with VRT. We bought some Technologic Systems TS-7670 industrial computers. We found the stock kernel (2.6.35):
- was flaky on the supplied units with regards to USB
- suffered buffering issues on serial ports at low baud rates (which played havoc with M-Bus)
A port to kernel 4.2 was trivial, and the devices in the field now run that.