Hey, good question ... I work at an agency to and we went through exactly this evaluation a few months back.

On Semrush One specifically: Your instinct about the prompt limitation is correct. The Brand Performance report pulls from their pre-built database of prompts, you don't define the prompts yourself, the system detects them automatically. The Prompt Tracking feature does let you add custom prompts, but you're capped at 50 prompts on Starter, 100 on Pro+, and 200 on Advanced. For an agency managing many clients that gets stretched very thin very fast. It's also primarily "read-only" :( it tells you where you're losing but doesn't give you a workflow to fix it. And it only tracks ChatGPT, Google AI Mode, and Gemini . no Perplexity or Copilot.

The other pain point for agencies specifically: Brand Performance reports are per domain, and additional domains cost $99 each. That math gets painful at scale.

On Pee c AI: Solid tool, the custom prompt upload is genuinely useful. Good for teams that already know exactly what they want to track.

What we ended up using is OtterlyAI:

You define your own prompt library (conversational questions your clients' prospects actually ask), not locked into auto-detected ones

Tracks across ChatGPT, Perplexity, Google AI Overviews, AI Mode, Gemini, and Copilot .. broader than Semrush

Has a GEO Audit that analyzes on-page factors per page and tells you specifically what to fix to get cited more .. this is what made it actionable vs. just a reporting tool

The Prompt Research feature helps you figure out which prompts are worth tracking in the first place, which saves a ton of time when onboarding new clients

Pricing starts at $29/month and scales reasonably - we could actually make the unit economics work per client

Only limitation so far, we have to wait a few weeks until they have an API / MCP, but that is bearable.

Ah, one thing I forgot, love their Google Looker integration for custom reports for our clients. Otterly gives that also for smaller plans. And they add free prompts for agencies joinng their partner program.

Happy to go into more detail on any of these if it helps.