I find that there are benefits to shooting mirrorless. First, there is no need to lock the mirror or shoot in live view to limit shake. Second, with manual lenses that you must focus stopped down, you get a bright, properly exposed image to focus upon in your electronic viewfinder. Lastly, as I do a bunch of focus stacking, the in camera focus stacking program of my R5, acquires the images in a fraction of the time as there is no mirror or shutter slap and I don't have any settling time. The result is that I have fewer stacks that are ruined by wind or changing light.
Aside from the cost of a new body
I see no disadvantages to shooting macro with mirrorless, with the possible exception of shooting fast moving subjects in which you may get a rolling shutter effect.
Irwin
Canon 5DSr, Canon R5, Sigma 180mm, Canon RF 800, Canon RF100-500 L, Canon RF100mm, Canon 65mm mp-e, Laowa 2.5-5x, Canon RF 24-105 L, Zeiss 60mm, Sigma 50mm, Zeiss 24mm Luminar, Nikon 5x m plan objective, Stackshot focusing rail, Novoflex bellows, Kenco tube sets etc