The newest firmware updates for still supported Samsung drives enabled queued TRIM support as part of SATA 3.2 enablement, but the feature is supremely buggy. Issuing a queued TRIM at the wrong time and the SSD controller locks up, possibly eating some data in the process. And who knows what happens when it seemingly works.. Samsung support and other public channels will likely still deny this, but a fix should be in the works according to my sources.
Luckily for Samsung only Linux and possibly Windows 10 (but thats still beta) has started making use of this feature. Linux recently blacklisted the feature on Samsung 8*, but it has just started to come out in distributions update channels, and install media also need to get updated.
For the 840 EVO this was extra bad luck; the new firmware is required to get usable read performance, so everyone is updating, getting broken queued TRIM instead.
For the 850 Pro I've got 3 shipped with broken firmware from the factory from two different suppliers a month or so ago.
Until the dust has settled stay away from Samsung SSDs.