Where to find competitor reviews (and what to do with the angry ones)
G2, Capterra, app stores, and Reddit. Read only the negative ones, and only for repeated complaints. Those are your openings, written by their own customers.
Your competitor's customers will tell you exactly how to beat them, for free, in public, in detail, at some length, while angry.
Almost nobody reads it.
Where they are
For B2B software, G2 and Capterra carry the volume. These have real detail and a rough sense of who wrote it, though be aware that vendors actively solicit positive reviews there, so the top of the page is somewhat cultivated.
App stores, if there is a mobile product. Ratings there are blunter and angrier and often more useful for exactly that reason.
Reddit is the good one. This is where people say what they actually think, to peers, with nobody from the vendor watching and no incentive to be diplomatic. Search the competitor's name plus the word "alternative" and read what comes back. It is the least-monitored, highest-signal source in this whole list.
And churned customers. If someone left a competitor and came to you, you have a primary source sitting in your own client list. Twenty minutes with them beats a month of reading reviews. Ask what specifically broke.
Read the bad ones only
Five-star reviews are useless to you. "Great product, love the team" is a nice thing for them and tells you nothing you can act on.
Go straight to the one and two star reviews. That is where the product truth lives.
But do not read them for satisfaction. Read them for repetition.
Repetition is the whole method
One furious review is a person having a bad month. It could be about anything. Ignore it.
Nine reviews complaining about the same broken import flow is not nine bad days. It is a structural weakness, one their team almost certainly knows about and has been arguing about internally for a year without fixing.
That repeated complaint is the most valuable thing in your competitive research, because it is verifiable, specific, and it came from their own users rather than from your marketing imagination.
Turning it into something usable
Here is the part that separates this from doomscrolling.
Take the complaint that repeats. Make the opposite of it a claim you can prove.
Not "we are easier to use." Nobody believes that from anybody, and it is not checkable. Something narrow and true instead: their users keep saying the setup takes three weeks, and yours takes an afternoon, and here is a customer who will say so.
Specificity is what makes it land. Their reviews already did the research for you. You just have to read them and then be honest about what you found.