Fixed-wing advantages over flex wing .
With regard to microlights, fixed-wing or tri-axis refers to types of aircraft with a similar style of construction as a conventional aircraft as opposed to the "hanglider" wing and trike arrangement. This is known as a flex wing type and is frequently what people first think of when they hear the term microlight.
In general aviation terms, rotary wing aircraft are helicopters and fixed wing is for every other type of aircraft.
There are a set of rules and definitions to describe exactly what a microlight is which include maximum take off weights, wing loading, fuel capacity, number of seats and probably with a few other criteria for good measure.
Nowadays it can actually be very difficult to tell a microlight from a group A aircraft and vice versa. Some manufacturers even have options for home builders to modify the same airframe to fit either category. The Cook Flying Machine, CFM Shadow springs to mind here.
You may well wonder why, but given the rules for obtaining and maintaining a pilot's licence this begins to make a lot of sense.
You cannot fly a microlight over a built up area and Prestwick airport for example will not allow microlights to land even though the approaches are clear of the town.
The light weight, ease of maintenance, economy, practicality and fun of the type appeals to many group A pilots. Therefore a few simple changes to construction really opens the market.
The latest generation of tri axis machines are fast, practical and comfortable often of GRP construction rather than tubes and fabric braced with wires. Even the ones that are still tube and fabric are so well designed and built that they look far more like "real aeroplanes" than ever before.
A PPL(A) pilot could fly a fixed-wing microlight with little need for conversion although the lack of weight and relatively high drag has caused a few problems on final approach before now.
Flex wings on the other hand are controlled very differently to the point that the controls appear reversed. Pull to dive push to climb, push out right to go left etc. It is more complex than that obviously but you get the idea.