Thinking about a trip to Greece next summer, flying one way Athens to Rhodes and maybe a ferry back. We did Naxos one year and there was a 5 hour ferry ride I found a bit tedious but maybe an overnight might be fun? Comfy cabins? friendly passengers? decent food? I do assume no pool. Or in reality is it just sooo much better to fly? Thanks.

If you found a ferry ride from Naxos through the labyrinth of Cycladic Islands up to Cape Sounion and finally Piraeus tedious, you will certainly not enjoy a ferry ride from Rhodos to Piraeus. The boats are similar (same company) and the food in the self service restaurant is nothing to write home about (rather high prices for rather bad quality). Cabins are usually clean and functional. Usually no evening entertainment (except Greek TV programs).
The biggest attraction is the landscape: Greek Isalnds and Turkish peninsulas up to Kos (afterwards it will get night).
If you take a overnight fery, you may have to leave your hotel around 4pm and will arrive in Athens city center around 9am the morning after.
If you book a flight leaving RHO around 18.30, you will have to leave your hotel at 4pm too and will arrive in Athens city center around 9pm.

You would save one night’s hotel bill, but other than that I can’t think of an advantage of a ferry over flying back to Athens. No, there won’t be a swimming pool—these are car ferries, not cruise liners.

I have taken overnight ferries to Crete and Rhodes, not by choice but by necessity.
Tedious and tiring, the only advantage is arriving early morning on Crete or Rhodes which is useful with onward travel in mind.
Working ferries not a cruise ship.
When I went before the smoking ban, clouds of smoke in the lounges and very noisy all night (Economy, no cabin) took to crashing out elsewhere in corridors or restaurant areas for quiet and not so much smoke.

Some of the more interesting ferry routes are some of those to Rhodes if you stop and stay on the islands on the way intentionally.

For a simple point to point travel you would do better to fly.

I took the overnight ferry from Piraeus to Crete back in 2006, but I booked a cabin. Functional if basic and I slept well.