It used to be that a target's value was highly relevant in deciding risk - a company like a bank would be a bigger target than say one who made bricks, and so hackers would not waste their time on the latter so much. In recent years, the spread of bot-nets means that one hacker can attach thousands of locations at once, looking for vulnerabilities. They then benefit (often) by ransom, rather than using the info themselves - the brick company may be just as anxious to get its files back as a bank and maybe more likely to pay up as they are unregulated. So I tend to look at things a bit differently than Victoria mentioned.
I think one must presume any given system may be hacked, and start asking yourself what the damage is to you and how to mitigate it. While there are some photographers where theft of a photo (e.g. to be resold or reused) may be terrible, or an invasion of privacy, but for many (most) the issue is not so much that someone will get your photos, or resell them, but that they may make them inaccessible to you.
One aspect of the cloud is that many services automatically sync changes. So for example, your own PC might be infected, corrupt your photos (encrypting, deleting, etc.) and in turn those copies may flow to the cloud quickly before you realize you have a problem. Ransomware can be a real problem in this regard, often accidentally, or purposely ensuring that attached copies of your files get encrypted as well.
In this regard, while one should do proper diligence on the security of any cloud service, you should also just assume it might be hacked, or that your computer might be hacked, and be sure you take other precautions, e.g. to protect against ransomware, you must have multiple, versioned, OFF LINE backups. To protect against theft (e.g. if you have a very private, or very valuable-for-reuse photo), consider doing your own encryption on your own computer, so anyone stealing it cannot make use of it (do NOT depend on encryption after it gets to the cloud, as that implies the cloud provider has the keys).
There is another danger as well, and that is that services like Adobe might or might not take adequate care in moving the files around. You might trust your originals to such a vendor only to find that through simple mistake, not malicious intent, the files become corrupt. Having good backups for this is the primary protection, but one also needs some way to NOTICE such problems. The DNG checksum checks are good in Lightroom, though do not apply to other files. Google "Bit rot" if curious. Personally I think people should put more pressure on Adobe to address file integrity checks end-to-end (even starting with card ingestion), but that's another discussion.
Sorry -- long answer, but my recommendation is to assume you will one day get hacked (whether your PC or the cloud) and make sure you have protections to mitigate it. Trying to decide if one very secure service, like Adobe, is more or less secure to say Dropbox or Google is an exercise in frustration -- they are all "secure" and they can all be backed.
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