Well I definitely wouldn't put it as unacceptably incomplete, after all it is a very rare event that you're forced to underpromote into a knight, unless you're fooling around.
That said, there are cases in real games, where you actually have to promote knights (again very rarely, but still every now and then you see one of those), because e.g. you have to promote with a check (otherwise you may get checkmated), so if in such scenarios it's only with knight that you can check, then you really need the knight-promotion option. A random example:
[Title "1.e8=N+ Forced knight promotion"]
[fen "3K4/4P3/2qk4/8/8/8/8/8 w - - 0 1"]
1. e8=N+ (1.e8=Q Qc7#)
Here's another one: (match between Tomic vs. Winzbeck)
[Title "Forced N or B promotion else stalemate"]
[fen "3rb2k/2P4p/6pP/p2B1pP1/Pp2pP2/1P2K3/8/8 w - - 0 1"]
1. cxd8=N (1. cxd8=B) (1. cxd8=Q)
On the other hand, it may also happen that if you promote into anything else than knight, you stalemate the position, so again knight-promotion needed. You'll find tons of examples from this chess SE post and links therein.
Whereas if you take online blitz games as example, then most often (for time controls < 5 mins at least) players set their automatic promotion to queen so that in case they're in time trouble and have to promote, they can premove it.
All in all, I would say that you should try to implement it for completeness, and if it turns out difficult for you, then don't worry too much about it, no big deal.
I actually do recall a very fun game between Hikaru Nakamura and Boris Savchenko in the previous world blitz championship, where Nakamura had to underpromote into knight with check in order to avoid a mate in 1, here's the clip, it's only 2 minutes, watch it from the start, but if you just want to see the promotion moment, here it is.