Everything in holster design involves compromises among the four basic factors of comfort, accessibility, concealment, and security. Whenever one factor is emphasized there will be compromises in the other factors. There is no such thing as the "perfect" holster design for every user, every handgun, or every situation. There are only compromises. Only the user can decide which factors are most important for his circumstances and how much compromise of the other factors is acceptable.
The IWB-style holsters ride between the user's body and the trouser waistband and belt. Retention is provided primarily by pressures exerted on the holstered weapon by those boundaries.
A thumb-break retention strap places the snap-strap attachment between the user's body and the rear surface of the semi-auto pistol slide, a point at which considerable pressure is exerted. The degree of pressure applied can vary depending on the user's body position, activities, and other influences (such as furniture or automobile upholstery, etc), and in some circumstances it can become very difficult or impossible to release the thumb-break and draw the weapon.
The overall size and very short length of your Ruger LC9-S tend to exacerbate this. Any IWB-style holster for the very small sub-compact pistols must place the bulk of the handgun so low into the waistband that accessibility is much compromised, and the addition of a thumb-break compounds the problem.
For these reasons I always declined to provide IWB-style holsters with thumb-break retention. Of course, my holster-making career spanned only 43 years (including 24 years as a LEO), so I am certain that there will be others with different opinions to offer.