Hi @mkweber,
Thank you for the reference! I've not used NemaBase, but based on a quick skim of their paper it appears that NemaBase is curated to optimize nematode taxonomy assignment. I think it should serve you well.
It wouldn't hurt to compare to SILVA 138.1, you'd have to follow the SILVA RESCRIPt tutorial yourself to see if there have been any changes, but I doubt there would be any significant changes.
You can even use GenBank data to make your own database too, and compare... for example there is this tutorial, and this one too.
Just make sure that the NemaBase, or any "specific" database you make use of, has "outgroup" taxa. That is, a collection of non-Nematode sequences within the reference database. If there are no outgroups, then quite many sequences will be classified as "Nematodes" when they are not. This is often overlooked.
But I think you'd be just fine using NemaBase. 