You can use Calibre to convert with kindles, but some documents don't convert well without a lot of effort/tuning and you really shouldn't have to. Spend your money elsewhere.
The biggest downside of Kobo for me has been Rakuten's update cadence. The system updates opt-out doesn't actually work from what I can tell and my device only comes out of airplane mode once every few months. Every damn time I do so it has to download an update, lagging out for a long time, and changing the UI in exciting new ways.