Since others have already answered in a specific way regarding Sankara himself, I want to address the question more broadly:
This is essentially one of the key matters which divides MLs (and most other Marxists) from anarchists and libertarian-oriented forms of socialism.
Just like Sankara, I am an ML so my bias is going to be present in this answer although I'll try to be even-handed about it.
The problem inherent to syndicalism is something which Lenin defined as "trade union consciousness". Essentially, if you have unions that are specific to workshops or businesses then you have workers with a larger stake and expanded power but ultimately that stake and that power exists within the capitalist mode of production. Broader unions that are across an individual trade or industry, or even cross-industry unions also suffer from the same problem.
Unions tend to be very narrow-focused and their class character is not necessarily revolutionary, and in fact most are not - some are even what are known as "yellow unions", the absolute worst of the bunch. As such it's naive to uphold unionism dogmatically as a revolutionary strategy because unions are inherently limited in their scope.
I'm going to completely gloss over the late AFL-CIO's counter-revolutionary role on the national and global stage here because you could write an entire book on that particular subject.
Poland's Solidarnosc was instrumental in the overthrow of communism in Poland and ultimately it created the conditions for its own steep decline.
The worker's councils in Yugoslavia were critically flawed and due to their parochial scope and their vested interest in protectionist policy, the Yugoslavian economy was critically hampered by this situation, with unemployment running at around 10-15% at any given time which is astonishing for a socialist country. The Yugoslav model is complex, as are the causes for its downfall and you could write a series of books on both however it's not unfair to say that the economic conditions of Yugoslavia were completely unsustainable and Yugoslavia's approach to worker control was part of this problem. Likewise, the stark ethnic divisions that ran through Yugoslavia also existed on similar lines of economic development/underdevelopment in the country which was disastrous for Yugoslavia and what came from its fall. It's beyond my imagination to be able to come up with a way that Yugoslavia would have been able to cross those ethno-economic fault lines that existed in order to bring prosperity to the backwater regions while maintaining a high degree of worker control like they did.
In the Spanish Republic, in areas controlled by the CNT/FAI, workers rights and worker's control was rapidly expanded but this often led to immediate and steep declines in productivity (and, perhaps by coincidence, the unemployment rate in the Spanish Republic ran at about the same levels as Yugoslavia. The fact that they were in a situation of civil war against fascism and they hadn't fully mobilised their working populace absolutely boggles my mind tbh.) Under these economic conditions, worker's rights and worker's control was often rolled back significantly, with one of the most obvious examples being the establishment of a judge, jury, and executioner role to enforce labour discipline known as the "Distributor of Tasks". These officials answered only to the highest levels of government and they could summarily dismiss or imprison workers for very minor infractions such as not being productive enough at work, for turning up late too often, for being on sick leave without justification, and for having an attitude of defeatism(!!). There's an excellent, comprehensive work in two volumes by Robert J Alexander (who is very sympathetic towards the CNT/FAI) on the economic conditions of the Spanish Republic titled "The Anarchists in the Spanish Civil War” if you want to read more about this topic. I'll spoil the ending for you though:
So with these socialist examples of the shortfalls of unions, there's a good case for why MLs have lukewarm sentiment towards them as a be-all, end-all revolutionary program.
There's more to be said about ways that unions can be played off one another, how they often become willing accomplices to arrangements that screw over workers or the masses, and so on but better writers have done a more comprehensive job of this than I could. But I will give an example here: consider the importance of universal healthcare and its popular support, even in a country like the US. Where there has been a push towards universal healthcare, historically there has typically been resistance to this by none other than healthcare unions and healthcare associations (which effectively function along the same lines as unions for the purposes of this example). Why? Because healthcare professions tend to face significant threat to the profitability of their industry due to the introduction of universal healthcare. This is one of the most obvious examples of how trade union consciousness can ultimately be not only counterrevolutionary but downright reactionary.
