Navien does have a "bridge" type bypass for using the no-dedicated-return type of gadget, and they do have settings to accommodate their device.
What they do have that is actually great, is the NPE A series with the mini tank to keep a bit of hot water ready to go, and eliminate hot/cold sandwiching.
They have great tech support but only for actual plumbers.
I am a fan and there are excellent pros who have had a few bad experiences.
Local support options are important, so look up on brand website to see who is listed as NSS (Navien service specialist) in your area, or verified as trained at least, like me, but not NSS. (NSS requires accepting their poor reimbursement rates under warranty).