The only drawback for using ViaBTC is having limited number of transactions can be accerelated for each day, so you can't really expect them to able include all of your transactions.
Who knows if they are doing that to make people use more of their paid services because if they continue to help, it will be as if the are cheating those who are paying high fees for including their transactions in any block they mine successfully.
Is there a particular time to know when they accept unconfirmed transaction in their website because I tried to test some transactions and it's saying submission are beyond limit all the time.
These bitcoin accelerators will be only helpful when someone really messed up when trying to create a TX and broadcasted with the wrong fee and really in need of getting them to be confirmed as soon as possible but they become obsolete since the implementation of RBF and even now non-RBF TX can be broadcasted again with high fees since nodes enabled full RBF features so their paid service may no longer be used by anyone.
If you pay a wrong fee and RBF enable, there service is of no use as it can be bump but you got me confused where you said even Non RBF transaction can be rebroadcasted again since nodes enable full RBF.
What I know about RBF is that if you don't enable it in your transaction, don't expect to bump your fee regardless of the mode you are using, it will remain unconfirmed for long time until then few comes down.