How can you say a developer is bad without being subject to a defamation lawsuit? If I add "this is my opinion, you can come to your own conclusions", does this still leave me vulnerable to a defamation lawsuit? How so? If I am not expression a statement of fact, and I deliberately say it's my own opinion, how can I still be?
As others have said, this is jurisdiction dependent. The following is from a US perspective.
In short, you can't say " a developer is bad" (which a reasonable person would interpret as referring to the developer's skills, professionalism or character), and not risk a lawsuit.
Merely labeling something as your opinion, does not make it statement of opinion. The difference between fact and opinion is that a statement of fact is falsifiable according, while an statement of opinion is not. This is especially relevant because under US law, proving falseness of the offending statement is a required portion of the plaintiff's case.
Note that the above statement is not only potentially defamatory, but (in my non-lawyer opinion) seems likely defamation per se (which means that the plaintiff does not need to prove actual damage from the statement).
For example, in TRONFELD v. NATIONWIDE MUTUAL INSURANCE COMPANY (https://caselaw.findlaw.com/va-supreme-court/1194415.html), the statement that a lawyer "just takes peoples' money" was found to be inherently defamatory per se.
That's not necessarily to say that a lawsuit would necessarily prevail (the developer would have to prove the statement, that they (and/or presumably their work and/or skills) are not "bad", on the balance of probabilities), but it would clear the bar that the statement was not an opinion, and so could not be dismissed on those grounds.
One might be legally safer, if one very much wants to complain about a developer, to mention easier to verify true statements (e.g. if work was not completed to specification, or to contracted schedule), or genuine unverifiable opinions (e.g. "I was not satisfied with the work; I thought the graphic was ugly", because ugly is subjective).
Note that there are not "magic words"; context matters. The best way to avoid a lawsuit is to avoid negative comments.
Also note, that depending on the state and specifics, Anti-SLAPP laws might come into play (potentially in your favor).