There is no guarantee that add-ones sold in the Marketplace will work after sim updates. However, if they do break, you will be waiting forever for fixes. As an example, I’m still waiting for Flightbeam KDEN SU5 fixes to propagate to the Marketplace. Had I known about this ahead of time, I would have never bought from there. You also get no proof of purchase (since you buy “credits”) and some products are different if you buy in the Marketplace; i.e. Latin VFR AreaX can’t be configured, etc.
As far as Marketplace alternatives go, Contrail is another good option.