I've been trying this Gemini 3.8 Flash for a day. Looks not much different than Gemini 3.7 Flash in my use case: I have Codex (gpt-5.6 sol) write up a design plan to implement a feature or refactor a portion of a system I am building, and have Claude (Opus-5) and Gemini (3.8 Flash) review and critique the plan, until all problems are addressed by Codex and approved by the reviewers; then have a cheaper model of Codex (gpt-5.6 luna) implement the plan, and still have Claude (Opus-5) and Gemini (3.8 Flash) review and critique the implementation, until all problems are addressed by Codex and approved by the reviewers.
The result is the same as the previous Gemini 3.6/3.7 Flash days: Claude could always note much more problems in Codex's plan and implementation than Gemini could - the ratio is like 10:1.
I occasionally switch the roles between Codex and Claude, and result is the same, Codex could always catch much more problems in Claude's plan and implementation, than Gemini could.
So I am guessing in a relatedly complex codebase, Gemini is much less effective in acting as a guardrail (or a senior engineer/team lead) than the other SOTA models.