I have been using a Spektrum 2.4 plug in module without any problems on my Futaba 9C for about 9 months now. I have only been using it on my low dollar bind and fly stuff and only recently went 2.4 on some 51” size planes to cure some servo issues. So all is still good and confidence is higher but now I am putting together a 35% Extra and need to go 2.4 AR9110 9 Channel on unregulated 2s lipos and only using a total of 8 channels. A couple of people wanted to steer me to a non-module type 2.4 radio like there is some kind of issue using the modules and not the direct wire radio. Is this really a potential problem?
Lastly the Module I have says its 8 channels; I guess I lose one in the conversion on my 9 channel radio. Will I still be able to use the 8 of 9 channels in the AR9110? Thanks.

The only issue Ive heard is that the pins/sockets where it plugs in ay get dirty and/or loose and cause connection problems.On mine I noticed that it makes the voltage indication flicker.And its really really slow.

You'll be fine with the module. And "slow" is a relative term. If you're just doing sport flying you'll never notice the faster response time of a system with built-in 2.4. And we're talking milliseconds.

I've been using the same set-up for about 8 months without a single problem.

OK Thanks. I will stick with this set up for my 35%.
So there's no issue using a 9 channel Rx on a 8 Channel Tx? You just would only be able to access 8 channels?

Yes, that's correct you only get the 8 channels that are active.