Mk1 is spot on as few others.
What annoys myself, is the change to 61.75 regs few years ago limiting the validation to ONE issuing country and not 100% sure if also not one class/category. Haven't tested/researched that. Some FSDO guys may not have known for some time, as I had to show the regs to guy 'giving up' my Czech glider validation to have SEP VFR validation from elsewhere done, towards FAA IFR. Then can't fly some gliders other than solo and as solo, in USA, no passengers. Not huge issue, but silly for semi-experienced pilot flying on older student cert on medical and solo sign-off.
Another thing to be aware is that FAA should/may refuse doing NEW validation once you obtain full/standalone FAA license in same or different category. I kept mine and strictly speaking one can have aeroplane validation and later do checkride and keep/not give up validation.
If every planning to do more training or get FAA commercial, do so rather earlier, as it's house of cards keeping FAA specific ratings on validation but no full faa private or commercial.
Yes, Mike, it's bit fuddled up/not obvious and legalese, but yeah, as mentioned. My IACRA was messed up and 8710 on paper was needed as the system wasn't set up for someone having rotary commercial and doing instrument on other category on private validation.
If having VFR PPL/CPL from abroad, 61.75 validation can be 'license to tack on' FAA IR (US test passed) after checkride with FAA DPE/FSDO inspector.
For foreing PPL/CPL with same issuing authority IR, one only needs to sit the 'IFP' Instrument Foreign Pilot written through PSI (ex Lasergrade) or CATS locations - basically most FBOs/flight schools, for 150 USD (last time I checked) with possible AOPA 10 bucks discount - ask for it and book over the phone, not with the location, who may not mention it. That is, if you are AOPA or similar member that provides the discounts or fancy supporting US GA advocacy etc.
Every validation needs FLIGHT REVIEW in USA/with CFI even if current 'back home' before exercising the privileges, ie PIC on flight. Shouldn't be an issue, as before renting a/c you're likely to do check flight or two with company instructor. The FAA guys will likely remind you of it doing the Temp airman cert.
As for IPC and instrument currency after getting foreign IR transferred to validation either at initial request or later on (but need the verification letter 6 months old or less, silly I know), I'd have to check the regs. I only have uncurrent FAA IR on aeroplane private from elsewhere and full FAA heli ratings. Using the validation as basis for flight test (if not having full FAA licence later on) ie for commercial, also requires current validation letter, even if not redoing/having the piggyback re-issued. So are any changes to ratings, adding ratings or changing address.
The keeping current/building hood time as Mk1 mentions, falls under some specifics, so best be aware if tyring to do so. While under hood, the 'seeing' pilot is the Acting PIC responsible for collision avoidance in VFR and needs to be rated/current. If in IMC, also instrument rated and current. The logging is FAA thing, limited advantage to European pilots needing time for further ratings, but doable with some caution and caveats.