DD (currently in yr 9) is in the Welsh stream of a bilingual secondary, so a bit of a half-way house. She studies maybe 2/3 of her subjects through Welsh (basically all the humanities, plus PHSE, music, maybe another?).
We had the choice of English stream in the same school, or fully Welsh medium in another school (although actually to complicate matters she would still have studied science/maths in English). I know lots of friends with DC at the Welsh medium school, so have a reasonable view from both sides.
Homework has never been an issue - TBH at secondary, the actual doing of it is their problem - though in yr 7 we did a fair bit of reminding to make sure it happened. Both in dd's school and the fully Welsh medium one they do lots of vocab work to make sure the pupils have the appropriate knowledge, they're well aware that many/most come from non-Welsh speaking homes.
There were specific school related issues - not language based - that put dd off the Welsh medium option here. They do (as is usual I think for Welsh medium) get extremely good exam results, and IMO they are overly results focused, which means that other things suffer. But of course that's a school-specific issue.
Most of the DC I know who are in the Welsh language school are perfectly happy there. The one language thing that I know has been a problem for some dd's friends (and dc of friends of mine) is that they are very, very hot on Welsh only for the pupils,. That includes situations where, to give examples, a pupil is suffering severe bullying, or has major health problems and is in a meeting including a parent who doesn't speak Welsh. Again, that's a pastoral issue, and I suspect would differ across different schools. There's also a certain amount of Welsh vs English tribalism, though I think it varies greatly by school year.