I tried to replicate your case:
i followed two ways:first i used FLOW tool to transfer rectangular grips on cylinder(red)
second i just placed rectangular grips on the cylinder using POLAR ARRAY tool (green)
here on top wiew:



you can see difference: FLOW tool make a "distorsion" on rectangular grips when flowing
doing a close-up you can see different angles



second (green) give a better condition to the FILLET tool to work...more "opened"...

i tried to fillet flowed grips after BOOL-UNION only with a small radius,but not enough
filleting green placed grips after BOOL-UNION gave more chances




i used G2 Fillet 1mm radius,i did a lot of grips so the file is too heavy to post (100 Mb more or less)
selecting all and do the Fillet in one stroke require too much time for calculation so here,on picture, i just select 6 grips,as you can see
hope that Michael or someone else can explain better than me what i tried to show

M

Why not just calculate just one unioned box fillet and make an array fill around the cylinder?
maybe more speedy calculation!

Ouì ...here i followed "normal" workflow:
-make a filleted grip
-do copies with ARRAY
-melt together with BOOL-UNION
-fillet contact surface between grips and cylinder

my target is always to obtain a clean solid,your suggestion is OK Pilou
other shortcut could be to trim cylinder and rectangular grip and do a BLEND,than array all etc. etc...
i just pointed attention on "distorsion" that FLOW sometime do.You know..FILLET need good conditions for working,first of all avoid areas with seams ;)

Thank you for your answers ... but I'm sorry, this time I couldn't post my 3DM File - I did solved my problem with PAINTING AN DISPLACEMENT-MAP ...

but I will learn from your answers. Thank you so much ;)