Hi, I am Emglish and speak English to dcs and to dh. Dh is Duthc and (mostly) speaks Dutch to dcs, we live in a Russian speaking environment, so they get that when outside/with friends and ds (aged 6) is now at Russian school.
I am also a teacher who has taught in several settings with high non native English speakers, and done several courses on supporting ESL kids. When I got marries dh and I spent a lot of time researching bilingualism, to see what we should do.
All of that is just to say that the following is backed up by a bit more that just experience.
I think there are some important issues here
First there are loads of types of bilingualism, eg passive bilinguals who understand a language but don't speak it, bilinguals with major and minor languages, bilingual one language at home the other the community, where 2 languages at home and so on. The point is, that it is ok not to end up with 2 perfect languages, there are still many advantages, and there are lots of ways of going about it, none of which are right or wrong, but I think there ARE some situations you should avoid.
I have come across families where there are 3 , 4 or 5 languages being used and the child is not actually proficient in any of them. (one child aged 7 was speaking to me in pretty incorrect heavily accented English. I asked her mum which language of the 4 was her strongest, and what did they speak at home (Mums English was pretty hard to understand) and she told me English was her strongest and they spoke English at home) This was also something highlighted in London where they were finding that Bengali kids (especially girls) were leaving school with good school English, but with vocabulary limited to the things needed at school, but also were not really proficient in Bangali eg they could not read and write Bengali, and had vocabulary limited to their home sphere. It is really important for our kids that they have one (or even 2 or 3) languages where they can properly and fully express themselves. That is why One parent one language is popular, because they are getting lots of consistent correct input from that language, so hopefully they will end up being properly proficient in it. I think it is also good to remember that there can be too many languages.
One thing we discovered when we researched it was that the families where both parents understood both languages ended up continuing with the bilingualism for longer and were generally more successful. When one parent didn't understand one langauge, it became too hard. Because of that we went to Holland for 6 months so I could learn Dutch when we were first married.
The other thing is that our kids are not all good sponges and not all natural linguists, but we shouldn't give up just because of that. My ds had always understood Dutch well, but he always replied in English until he did 3 weeks in Dutch school aged 5. Then he finally started talking Dutch. dd1 is already using 3 languages aged 4. She does it naturally and instinctively knows which language is which and which she should use for which person. Ds refused to speak Russian at all, even though he has been hearing it since he was a baby. Finally we got someone to come and do 40 mins conversation class (basically playing games) with him every morning for a month. That broke the damn and he started to speak (aged 5.5) and then his Russian took off. Until this point he had been quite isolated, he wouldn't play with any of the other kids outside because he couldn't communicate with them. Once he started speaking he made friends and now is well settled into our community, and then started scholl, and his Russian took off. dd on the other hand plays with everyone outside and uses whatever words in whatever language she can get away with!
I do know people here whose first language is English but the kids are with a Russian speaking nanny all day. They basically speak poor English and poor Russian (nanny is sweet, but she doesn't really sit down and talk and play with the kids.)
My kids all spoke early and well, but I have heard that bilingual kids sometimes appear to be late speakers, with smaller vocabs. The thing is, if you have a vocab of 20 words in English, but you are bilingual, then actually you have a vocab of 40 words, and that is often overlooked when kids are assessed.
We do opol, but when in England the whole family speaks English, and when in Holland we all speak Dutch, when at a friends house I speak Russian to my kids followed by the same thing in English. When my dh is tired and he keeps forgetting to talk Dutch, I speak Dutch to the kids to get us going again.
I think it is terribly sad when a parent feels they have to give in and speak the majority language. Knowing another langauge gives a window into that parent's culture that you only get with language. There are things about my dh I only understand because I speak Dutch.