What has actually been tested
PC Gamer’s September 3 hands-on reports that the demanding Kyoto area makes Onimusha a roughly 30–40 fps experience on Steam Deck. Its central recommendation is a 30 fps limit instead of chasing occasional higher readings. We have not independently reproduced that test.
Do not confuse a reviewer’s use of the word playable with Valve’s formal compatibility badge. The Windows requirements and Steam controller features also do not, by themselves, certify Linux or Proton behavior.
Make a small, repeatable handheld check
Begin with one known section and note your SteamOS version, game update, chosen settings and power limit. Play through a fight, not only the title screen. Look for consistent motion and readable attack cues while moving the camera.
If the game exceeds 30 fps in one corridor, do not immediately raise the cap. Repeat the check where more enemies and effects are visible. A lower stable target can feel more predictable than a higher target with repeated drops.
Consider the whole portable experience
Read the interface without leaning closer, check your chosen controller layout, and try a normal save-and-resume session before relying on the game during travel. Keep a confirmed save before any sleep-resume experiment.
The appropriate decision depends on your tolerance for 30 fps swordplay. If timing at that rate feels uncomfortable to you, a favorable published handheld report is not a reason to ignore your own experience.