Unless you pirate, for most publishers, there's no way to buy books without DRM. If you buy books for Kobo, you are locked on Kobo.
Also, last time I tried, Kobo's phone app was just not good. There was no vertical scroll like Kindle, the animations were not smooth. The online store is also a bit weirder for me to navigate - but that might be just me.
And last, if it was not bought at their online store, it won't have sync between devices, AFAIK
For me the main difference I've found is that ebooks stripped of DRM tend to end up in EPUB form as the presumption is you're ensuring the books remain accessible on future platforms so you're going for an open and 'standard' format.
At which point it's then common to tidy up things like the book metadata (eg correct the genre or add a series identifier). That in turn means the DRM-free EPUB version becomes the better version, so the native and network-free support of side-loading EPUB books on the Kobo etc is a useful thing.
But once you DeDRM in Calibre, it’s literally one click to convert it to ePub. My workflow for removing DRM from all my Kindle books is literally to just use Calibre, the plugins and then convert. It takes the exact amount of time it would take for using a Kobo device.
Side-loading ePub on Kindle is now a thing btw with the personal document system. So you can literally just email files to your Kindle address.
Send to Kindle is a very convenient way to push ebooks to a Kindle device (though it needs network access of course). It's supposed to support EPUB too, unless it already does.
I never had issues side-loading EPUBs to a Kindle with Calibre (the de-facto ebook management application standard), so I don't think that's a problem.
Also, last time I tried, Kobo's phone app was just not good. There was no vertical scroll like Kindle, the animations were not smooth. The online store is also a bit weirder for me to navigate - but that might be just me.
And last, if it was not bought at their online store, it won't have sync between devices, AFAIK