Totally forgot about bitvest; tried it only shortly because the returns were super low during that time.
Better avoid bitvest unless you are okay with long waiting period with no clue whatsoever, hot wallet not replenished is one of the main issue that people face on bitvest.
Bustadice and Bustabit can be trusted but bankroll investment can be double edged sword so DYOR and come into a conclusion.
Yep.
What would happen if the platform in question would just.. Close?

Better to be safe and stable, but still afloat.
This is the common dilemma for those gamblers who would want to invest in a casino offering such service. Because even if the site is reputable today, we don't know what will happen in the next days or months. They can easily shut down their operations without warning. I've tried before but it was not btc that I invested, but certain tokens. And luckily, I withdrew before the site closed. When I feel that I can't keep up with the updates, I tend to withdraw the assets. And my decision was actually just right.
Reputable casino with bankroll investment feature wont scam people, lets say they are going to close the service then they will inform their investors so there will be a time given for the investors to take their invested money.
Of course the worse case may also happen, so when it comes investing in casino's bankroll, people should always invest what they can afford to lose only.
Not only that casino may turn into scam later but there is also a chance of casino getting hacked and the bankroll stolen.
This investment can be said as a good long term investment but the risk is always there.