We know a LAR kills scrakes on 6man HoE with 5 shots. If you have the 3-4 seconds it takes to do this, you're fine. In addition, LARs stun scrakes, so short of enthusiastic grenade users, you're safe. You have enough time to keep him stunlocked, even firing every other shot, so you can stunlock 2 simultaneously for example (or shoot gorefasts who're running past). Also you can 'catch' a scrake if you lose him and easily stun him again.
The Mk23 requires 14 shots to decap a scrake (I'm not sure if this will also kill it, cbb to work out).
This means you need to pre-damage it, then empty an entire clip onto it once it rages.
This is bad because a) takes longer since you need to reload, b) if it gets hit by overspray then you get an angry scrake on you during a reload, c) if you miss a couple of shots then you get caught in a second reload d) if you don't know about flinchlocking you're screwed e) if a trigger-happy medic decides to help you out with MP7M to the scrake's gut you're screwed f) if a gorefast comes up from behind you can't pause and kill it
etc etc
If you're doing this after you've received money, or a few waves in then sure, you can buy a second Mk23 (which would be safer but NOT safer than the LAR) etc, but could also sell+buy crossbow, EBR, etc etc and take it downhowever you want. I'm talking about on your starting wave.
Plus, with a LAR you have both weapons on different keys, with the two pistols you will have to keep juggling between Mk23 and 9mm, or risk running out of ammo with the Mk23.
Plus flinchlocking relies on good net connection, and good knowledge of the game. Bar using a katana, it's definitely a trick for the top players, whereas LAR can be used by anyone across the board.