The problem may be a combination of vape style and topper. When smoking we take hard short puffs. The same style will cause burned liquid because the flow of the liquid cannot keep up. Vape with long gentle puffs to see if it makes a difference.
The topper can make a significant difference. I have several iStick 20w. When using a plastic Aspire CE5 anything but the slowest gentle puff will cause the liquid to burn. The licorice I use burns the quickest. Aspire K1s, which have thinner glass and metal than the Aspire Nautilus mini, are in the middle as far as burning. I prefer the K1 for flavors such as Creme de Menthe as it is a flavor that I do not want to vape vigorously. The Aspire Nautilus mini is a perfect match for the iStick 20w. A good balance by allowing a relatively strong steady inhale. I think the volume, thicker metal and glass absorb some of the heat. The adjustable air flow certainly helps. It may also be the design of the Nautilus coils. Overall, more of the ECF iStick member may be using the Naut. mini than any other topper.Some of the experienced people on the Eleaf iStick thread are reporting success with the Eleaf GS Air.
Just because a product may be considered older technology does not mean it is junk. The latest may also not be the greatest. When using these you may need to adjust your style more than you would do otherwise.
The heating problem of the iStick 20w has been fixed on the iStick 30w. When set a 3V the 20w is putting out 4.1V. It levels out as the voltage gets higher.The 30w also has better 510 threads and pin.
Aspire is coming out with new coils. They have larger air holes and cotton wicking. They are just now entering the US market. There is nothing on the boxes to indicate they have the improved coils. If you go with Aspire get one box of coils. Then, watch sites to see if the vendors note they have the new coils.
For everything iStick:
http://www.e-cigarette-forum.com/forum/variable-voltage-apv-discussion/601111-eleaf-istick.html
GS Air:
http://www.e-cigarette-forum.com/forum/clearomizers/642824-eleaf-gs-air-fan.html
ETA: Eciggity reportedly has the new Aspire coils.
Eciggity!