As it stands, you can read ebooks on a computer, an ereader and even some mobile phones. However, with more companies entering the market, it becomes difficult to keep track of which ebooks each device is capable of using. Some devices use files that are incompatible with other devices, restricting your library to only a few devices. Furthermore as innovations in computer sciences continually open up more efficient ways of doing things, it may be that the file type used by most ebooks may become completely obsolete in the future. Outdated software may force you to track down your entire library all over again. Also, ebooks are much more difficult to share with friends compared to paper books.
When reading on paper, light reflects off the page and into your eyes naturally, however, with computer screens, the device projects light into your eyes, causing headaches and possible long term damage to your eyesight. And while the LED projections used by ereaders dampen eye strain, it still has an overall negative effect.
Ebooks are just another file on a hard drive, and they often depend on the Internet to transfer from one location to another. A virus may invade your computer or you might accidentally damage your ereader or hard drive. Without a backup of your files, you could lose your entire book collection in one fell swoop. Moreover, third parties can monitor or even hack your files. You may fall for a scam and end up with a different file than you expected.
As with other media that has moved to digital distribution, ebooks are often pirated. Writers work hard to create their art, and they deserve to be fairly compensated for their effort. The prevalence of ebooks increases the chance that their work will be stolen online or that they won't earn what they deserve. If writers are not able to earn their living, they won't be able to continue producing more work as ebooks can pose a threat to authors' livelihoods.