My only contention with text captcha apart from those of bots is that these require you to be able to read, understand and know the answer of the question, and that too in the language that captcha is being served.
This might sound like a nonexistant issue, but lets take the example of a country like India with a huge number of internet users. There are many who use the Internet without being completely literate in English or even their mother tongue. Use cases like checking exam results, train tickets etc require captchas to be filled and the normal deformed letter ones do well. I doubt text captchas will be accessible to them.
But then again, user base matters a lot too and that should be taken into account always!
Off topic: If you are in Germany, the ReCaptcha is displayed in German, even if your browser's preferred language is "en". However, ReCaptcha is now ingrained in muscle memory for people, so they automatically click traffic light/stairs/bus/cars/cycles/palms/hills/zebra crossing, whichever is shown multiple times without reading instructions. For textual ones, people will adapt soon by pattern matching, whatever seems out of normal is the answer :)
This might sound like a nonexistant issue, but lets take the example of a country like India with a huge number of internet users. There are many who use the Internet without being completely literate in English or even their mother tongue. Use cases like checking exam results, train tickets etc require captchas to be filled and the normal deformed letter ones do well. I doubt text captchas will be accessible to them.
But then again, user base matters a lot too and that should be taken into account always!