What caps are they and why do you think they feed back more?
AT-3000 series T-341B hand held mics with non- replaceable capsules. I suppose they might have a more open pattern than an SM-58 or perhaps the off axis response is not as smooth.
I have had 4x of them for 4 years or so and they are always more feedback prone than any of my directional wired mics. The feedback frequencies are all over the board from 125Hz to 8kHz and vary for each venue.
I normally use the 0dB or +6dB internal gain setting and low RF power. I do take time with gain staging and also ring them out but they just seem far less stable than an SM-58, Beta58, or OM7. They exhibit more handling noise also.
The only other wireless mics I have worked with much were Shure SLX and BLX with SM-58 and Beta58 capsules. I've had performers jump down into the crowd with these right in front of the mains and not had much of an issue with feedback. The AT's would never survive that.
On the other hand AT's do have sold reception and decent build quality.