All tankless HW heaters introduce a pressure drop, and sometimes the anti-scald features of shower mixers will interact with them. If its misbehaving only in the shower, that would be my first suspicion- see if it happens if you turn on a sink tap full-hot at decent flow.
Most will also have problems when the water pressure on the line feeding to it is low. When lower than ~15lbs or so the micro-boil on the water side of the heat exchanger becomes macro-enough to sizzle audibly and lower the efficiency of the heat exchange.
If you have crud, frog-eggs, crawdads or other stuff coming into the filter it'll clog quickly, and a larger filter upstream is in order.
If you have hard water there's a good chance you'll need to de-lime the heat exchanger with a vinegar-rinse every year or two. Other than that and cleaning the filter occasionally there isn't much to it- it works mostly... until it doesn't.
There's also some chance that within the operating lifetime there will be enough varnish built up on the flame detector that it won't start up reliably, at which point taking a scotch-brite, fibertex or similar very mildly abrasive scouring pad to it may be necessary. (Don't use steel wool or sandpaper, which will mar the surface and make the problem recur quicker, or even damage the detector) This is a DIY deal, but requires killing the power & gas feed to the unit and partial disassembly to get at it.
The Takagi help line has the reputation of being fairly patient and informative talking a DIYer through a diagnosis when their units are misbehaving, but I've never personally made the call.