They Tachanka more viable in Rainbow Six Siege, as the developers try to shake up the game's meta.
The changes, especially for the Shumikha Launcher, are pretty comprehensive. To address this, the developers are aiming to make the character feel more powerful, especially his grenades to encourage their use more frequently.
In a blog post discussing the patch notes, the developers say that when Tachanka was originally released with the rework, he was considered overpowered, but he still has not seen as much play as the developers would like.