Beemo is good at one thing
Beemo detects mass-join raids — dozens or hundreds of accounts arriving together to spam, ping or harass — and it offers that free, with no time limit. If raids are your problem, it is a well-regarded answer and the price is hard to argue with.
It is not the attack ScoutBot covers
A raid is a volume problem: many accounts, obviously coordinated, arriving at once. Raid detection works precisely because the pattern is statistical.
Moderator impersonation has no volume signature at all. It is one account that may have been in your server for weeks, behaving normally, that changes its display name and avatar to match someone your community trusts. Nothing about its join looks unusual, because its join was not unusual. Detecting it means comparing identities, not counting arrivals — which is what ScoutBot does.
Price
Beemo’s anti-raid is free, and its Max tier is about $4.99/mo for moderation commands and message archives. ScoutBot’s impersonation alerts are also free for up to three protected names; Pro is $49/mo or $490/yr and adds unlimited names, automatic safe-role coverage, avatar image matching, and automatic kick or ban.
Run both
There is no reason to choose. Both have genuinely free tiers covering different threats, so the cost of running both is zero and the coverage is strictly better than either alone.