In terms of ETOPS…

An Adequate airfield is one that is suitable to land on, in terms of performance etc.
A Suitable airfield is an adequate one, that also meets ETOPS weather requirements. Differs for dispatch & inflight.

Good luck!

My tuppence
 

Adequate is where you can get on the deck without injuring anyone and hopefully not breaking the aircraft.
Suitable has so many caveats and potholes which have to be addressed and have led to notable incidents when not.
Engine failure out of Malaga diversion to Madrid, aircraft lined up on approach necessitated a go around which is when they should have checked Approach climb tables as they had a negative ROC. Saved by VMC and some clever flying.
Diversion without considering nationalities of Pax and politics. Pax taken of aircraft and disappeared.
Enroute diversion on North Atlantic as toilets hadn’t been serviced in winter. Lack of any suitable facilities and aircraft stuck for days. Don’t ask about where the pax stayed.
Suitable airfield was 11 hours away and home..didn’t make it and airline were threatened with loosing their right to operate into said big country.
Suitable airfield deemed to be destination although they didn’t have the fuel to reach it. End of career for big cheese.
Suitable airfield didn’t include flying the aircraft out again; iirc aircraft had to be unloaded and aircraft defueled and flown out by special crew.
Friend diverted to military airfield and was arrested along with pax.
Another friend tried several suitable airfields until he found one that could change an engine and not look at his freight.
Severe turbulence and crew continued to destination although they could have made Bombay where the cherry picker they needed for the tailplane structural check was sitting on the tarmac was located and not in their destination.
Luckily nowadays you have people on the ground who can sometimes help.