I do express lashes and find them popular amongst some clients.
They shouldn't feel uncomfortable (unless they aren't removed after a few weeks) and I'm not sure what you mean about putting stress on natural lashes? Like SP lashes, the extensions should be correctly suited to the clients natural lash so wouldn't put any more stress on the natural lashes than a properly applied set of SP lashes.
I find it a great alternative for clients who want a glam look for a short period of time but maybe can't afford the more expensive SP lashes and cost of regular infills.
I trained with ABX beauty (who do the ilash express) for something else and found them to be great.
Just to add, they take me more than 20 minutes to do! I allow 45 minutes for an express set (application takes me about 35 mins).
My opinion is that Express lashes are a great addition to your range of Lash services. x