Is there any risk of legal action if I proceed and buy the .io domain?
There is always risk. The question is how much risk you can sustain which often also translates in how big of a budget you have to pay your lawyers.
And the question is almost impossible to answer in general, it hugely depends on the name (since
bigdata.io is already reserved, it is of course not this one that you wish to register) and the website (search about this famous
milka.fr case, a domain name which was used by an individual with a website with a purple background that was deemed by courts to be infringing the same name as trademark of Kraft Foods, as it sells chocolate under that name in purple wrapping). So even if you think you are doing everything right and not infringing rights, others may argue differently and again it hugely depends on who is against you.
The domain name industry is filled with trademark interests so issues like that exist since a long time.
In the gTLD world, ICANN invented the UDRP to allow out-of-courts resolution of disputes. Out of curiosity, you should read about it and how it specifically defines if a name infringes on another. Of course this is not in place of disputes going into courts, it is an alternative mechanism, supposedly faster and cheaper.
If you have your mind set on an
.IO domain, like for any other, you should carefully read the registry documents on disputes, and then also by extension the registrars documents you will sign to get an
.IO domain, as a registrar could as well plug the domain out if it believes to infringe clauses in its contract.
.IO the registry is at https://www.nic.io/
No matter how it is sold and marketed, this is still a ccTLD. Which means something that is often forgotten specially for these kinds of ccTLDs sold as if they are gTLDs: you immediately depend on the country laws
(and then of course you will immediately depends on the laws of the country of jurisdiction of the registrar you will use to buy your domain)
If you read registry documents, you can collect such elements of information:
5.1 The Applicant agrees that any dispute arising out of or in connection with the registration or use of a domain name shall be subject to the provisions specified in the Dispute Resolution Policy.
1.1 (a) the registration of the domain name and any other listing information, to the best of the Applicant's knowledge, does not interfere with or infringe upon the rights of any third party;
3.2.1 Any third party may bring a claim against the Applicant arising out of or in connection with the registration or use of a domain name or any other listing information before a WIPO (“World Intellectual Property organisation”) Arbitrator in accordance with the WIPO Expedited Arbitration Rules, As Adapted.
So you can see that this ccTLD, among some others, decided to let WIPO handle disputes on its domain names. It has a specific page about .IO domain names: https://www.wipo.int/amc/en/domains/cctld/io/index.html
Like in courts, nothing is all white or black, so complaints need to provide other similar references and cases to strengthen their plea.
Unfortunately there is no good website to search on past disputes just based on the TLD. You can start at https://giga.law/blog/2017/8/9/where-to-search-udrp-decisions and use links provided but there is no ideal solution.
Also not directly related to your problem, but maybe to take into account about .IO: