I found the same when replacing my reliable old IC A-20 with a new 8.33 IC A6. Range with the former was easily 15+ miles, with the latter I struggled to make myself understood at my home-base ATZ boundary, and even in the overhead at a local airstrip. I called those helpful people at Icom, they advised me my ancient intercom was incompatible with the new radio. New intercom, range back to 15+ miles. Just a thought.

I use the handheld as the only radio in my airplane, and have done for 19 years. I recently flew another airplane where the panel-radio battery died and I had to use my handheld with no headset (no adaptor to hand); utterly hopeless, I couldn't hear a thing above the cabin noise, even at idle on the ground. I would certainly recommend if you keep a standby handheld available, have a headset adaptor plugged in to it already - it's really not much of a faff to change the headset over. You may need a separate PTT as well though, as the adaptor seems to disable the transmit button on the radio.

"may need a separate PTT as well though, as the adaptor seems to disable the transmit button on the radio."
I managed with the handheld PTT. No separate PTT with the 8.33 Icom adapter. Simple headset-adapter-handheld, with no intercom, as only used in emergency.
My old Icom adapter had a PTT on it.
I get about 15 miles probably, but the 25 Icom got much more.

Not true really as 121.5 is still a 25Khz channel.

A hand held radio in a metal aircraft is almost unless you have a way of plugging your headset into it and you have an external antenna conection.

Still better than no radio...........

Plug it into an external antenna and you can have a radio with a very useful range, rathe than some thing almost usless.

"Not true really as 121.5 is still a 25Khz channel."
What we mean by emergency use is not declaring an emergency, but carrying on after a radio problem, using normal procedures.

My Icom worked fine on the one occasion I used it for real, 40 odd miles out, no headset plugged in volume on max and the engine running at 2,200rpm - I am sixty with dodgy hearing - I had to strain to hear it and maybe didn't get all the words, but I knew what they would be saying and so the parts I did hear made sense and all ended well.

And I'm guessing yours is not 8.33 Ebbie, after all, why would it be!

"Plug it into an external antenna and you can have a radio with a very useful range, rather than some thing almost usless."
Certainly if you're using it as your main radio. But I'm wary of fitting a splitting device on my aerial connection so that I could plug in the handheld if the main radio failed. It could enable interference.
And fiddling about with the main radio connection in the air is NO.

A long time ago when King released there KX99 hand held radio one of the accessories was a mechanical relay.

King understood the shortcomings of hand held radios as a back up system and so sold the mechanical relay that could be fitted into the panel and connected between the main aircraft radio and the external antenna. When needed you plugged into the device with a cable ( supplied with the unit ) And connected your and held to the external antenna ( as well as disconnecting the aircraft radio ).

The hand held using the supplied antenna was around 12 miles at best with poor signal quality, using the King mechanical relay system I tested it out to 60 miles at 3000 ft and had good quality audio, I have no doubt the maximum range was beyond 60 miles but considered further testing to be unnecessary for an emergency back up system.