The guns look far to modern imo
This is in the future, so technically the gun looks too old, unless you're saying it lacks the "retro-future" aesthetic in which case, it might look too modern. These things really stop making sense, after awhile.
Perhaps saying it doesn't fit in with Alien "used future" is more readily understood than saying too modern. The word "modern" has no place in an ahistorical, fantastical film like Covenant.