I've never heard anyone describe Joli-Bois primaire as a bad school, certainly not Joli-Bois maternelle. They are 2 separate schools and your children would be at both so be aware of that.
Firstly both Joli-Bois schools have surplus classroom space, very unusual in Brussels, the primaire has 16 main classrooms plus about another 4 in which could be used in the loft floor, but in fact only 13 classrooms are used by classes - 2 for 5 primaire years and 3 for I think P2 or P3. It means it always has spaces. Joli-Bois maternelle has less space but still it always has space.
Secondly and unfortunately for Joli-Bois, the nearest other schools are Mater Dei, Sacre-Coeur de Stockel and ecole communale de Stockel and those 3 schools are chocka block full by September. EC de Stockel is the "least" popular and has just expanded from 2 classes per year to 3 but still it's better situated geographically for parents commuting to work by metro and better situated for French children who live in Tervuren and Zaventem where there are no French schools. It's a simple matter of geography that 3 other very excellent schools are nearby so they attract ambitious parents.
I think it's true to say that the 2 Stockel schools especially are popular with English mother tongue parents so a disproportionate number there, count on around 10%, then another 10% of children who have access to English as a second language, then you have a substantial proportion of the Belgian parents who can speak English quite well too. Don't believe the cliché that French speakers are worse at speaking English in Belgium, definitely not the case in WSP and Stockel area. There are less English speakers at Joli-Bois but they are still there and you can expect more than at the 2 Uccle schools you mentioned.
Really Joli-Bois is ok. It's no worse than the 2 Uccle schools. It's "richer" socio-economically than both of the Uccle schools at 20/20 score, St Job is 14/20 and Messidor is 13/20. That is based on 2011 scores which you can find if you search hard enough on the internet - it's a raw score showing how wealthy the average child is who attends the school. Don't read too much into these socio-economic scores but if you find a school unusually high or low in an area, it can often be quite telling, such as one school in WSL which is way way lower than all others. A high score of 20 means that the children are socio-econically at the richer end however of society. Academic results in Belgium do tend to be a reflection of how well off your parents are.
Don't run away from Joli-Bois, just know the competition nearby is rather popular.