Is there any way to check if it actually works? I've used ViaBTC accelerator for a bunch of times and it always took hours to get a confirmation even after transaction was already accelerated. So in my case it was kinda impossible to check whether it could take longer to get it confirmed.
It is much better just to change the transaction fee via replace-by-fee instead of relying on accelerators.
ViaBTC actually works, take note that after submission, your tx wouldn't immediately be added into the next block mined by 'any' pool, your tx will be added into the next block that's mined by ViaBTC, so i believe the reason why you had to wait for sometime is because it took sometime for ViaBTC to mine a block after you submitted your tx. Check hosseinimir93's post below to know on average how many blocks ViaBTC mines with their current hashrate.
The only time ViaBTC can send your tx to other pools they cooperate with to be mined faster, is when you use their paid service, for free acceleration you have to wait for them to mine a block themselves. But yes, i agree that it is better to pay a suitable fee while creating your tx, or just use rbf if it gets stuck.