I thought of it as most modern motors(that you would substitute) will be more powerful and more reliable than the A series so you have power and reliablity catered for.
It seems more of a balancing act between power and complexity to me I mean Matt (The MiniMan) is doing a V8 mini and probably needs a very big shoe horn to get it in and it'd be a fair bit of work. On the other hand a 3 cylinder engine is easier to fit but probably just as reliable in standard form anyway.
The other trade off is how many people have done it before and how many of the problems have they found the solution to already. Just look at the Starlet conversion I imagine that quite a few of the people are doing this conversion because they know it can be done and have the assistance of Matt (minstar) and Brad(13sec) if they have any problems.
This is asside from the positive attributes of the starlet conversion (starting with it being a Toyota

Yes I am a toyota fan I think they make some very reliable cars)
OK my 2 cents...keep in mind I can't be bothered doing a conversion too
