The Icom A6 and A24 both have feedback problems when used with an external headset. They are widely used in microlights - but only because Icom have a strangelhold on the UK market and because earlier Icoms (like the A3) were very good ......
The reason they work on microlights is that they are normally connected to a proprietary intercom (Lynx, Flycom etc) and the intercom manufacturers put fixes in their system to get around the feedback issues.
Vertex airbands are very good, not widely used in the UK as they are not CE certified.