Yeah he might not be terribly consistent but I'd take a leg spin bowler who gets wickets anyday.He might have had a few poor games but 198 wickets in 40 games is still a terrific achievement considering he's been in and out of the team all the time.
That's 154 in 34 (at an average of over 30).
Seriously look at MacGill's game-by-game breakdown:
36-134-5 - looks good, but really it was just getting gifted 3 second-innings wickets when SA were going for a declaration - which we'll come across again...
43-113-9 -superb
42-168-2 - abysmal
42.4-130-4 - pretty poor
46-121-4 - reasonable enough
53-108-4 - pretty good
46-142-7 - superb
40.2-107-12 - magnificent
16-41-3 - excellent
22.3-84-3 - reasonable enough
41-95-1 - abysmal
40-132-5 - pretty good
21-52-3 - excellent
32-84-4 - excellent
36-172-2 - abysmal
67-192-7 - reasonable enough
65.2-174-7 - looks reasonable, but twice boosted by Claude Henderson's wicket
84-260-7 - very poor
85-226-5 - abysmal
43-189-5 - poor
47-151-4 - very poor
75.5-182-9 - superb
38.2-156-2 - abysmal
30.1-118-4 - reasonable enough
68.4-244-4 - abysmal
41.5-138-5 - looks reasonable, but twice boosted by Ashish Nehra's wicket
54-211-1 - as woeful as you could wish to see
38.2-143-5 - looks good enough, but boosted by 3 tail-end wickets
17-89-0 - abysmal
47-170-8 - superb
37-87-5 - excellent
29-102-2 - abysmal
31-69-2 - poor
35-135-4 - doesn't look too bad, but as with debut was boosted by 3 wickets when SA were going for a declaration
So you can see the picture: the odd really good game here and there, but those number just 10 out of 34. He's not really done that well. And he's incredibly lucky Bangladesh and ICC World XI games are considered Tests, because they massively inflate his average.