Note: somehow I've made the Cons longer than the Pros, even though I love this job... that might be because none except the first two of these Cons are proper Cons.
TL;DR this way of working is pretty unique, and so it has substantially different characteristics from most other jobs, which means that some people with preconceptions taken from more traditional employment might be surprised or even displeased if they took this job. Being new and different is not a con, but such characteristics warrant being mentioned.
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The downside to the incredible flexibility of working for DataAnnotation is that you are technically self-employed and uncontracted, and, this being such an incredibly dynamic and evolving industry, I enter each day with the tiniest bit of relief that the job has not completely evaporated overnight.
The only other real downside of this job is the lack of career advancement. There is what I might describe as "micro-advancement", which is the "qualifications" you can take to progressively gain access to more challenging and higher-paying projects. If you are good enough, this will eventually reach a (pretty lucrative) ceiling, during which time your basic position and role has never changed, but the portfolio of projects to which you have access has broadened and deepened immensely. Try putting that on a CV, though.
There are no company benefits, no pension contributions - nothing. This is not a criticism, really, because this is simply the nature of self-employment; it just feels worth mentioning because one might expect to have the normal benefits of contracted employment if working for a company.
Another factor that I, an introvert, adore, but which others might hate is the isolation of the work. Remoteness is inherent to remote work, obviously. Even the work itself, though, consists of you completing your tasks for your selected project entirely alone. This will really appeal to some people (myself included - I like people, but I like being entirely self-sufficient), but never actually working with another person might be a turn-off for others. There are communal text chat areas where participants in a particular project can communicate, and where Admins for the projects will answer questions about the project, but this is the full extent of the interpersonal connection you will experience while working (at least, at the current time of writing).
Yet another "con" which is really more a natural aspect of the work is that there is almost no structure or hierarchy of personnel. I adore not having anyone to report to (do understand, however, that your work will be reviewed, typically by your peers in other projects), but, again, others might find this utter fluidity and the lack of an immediate guiding hand daunting. As mentioned, there are Admins for every project, but their direct involvement with us workers is very offhand, consisting of their answers to our questions when we ask them. Admins will never ask to see your work or give you a performance review or call you in for a meeting (what bliss).