I think they are the preserve of people who can't afford to insure a V8 or TDi.
They will do the job they were designed for, on paper they can shift 3.5 tonnes like a "proper" Disco.
They have the Defender transfer box IIRC which will effect top-end.
They can't give you TDi economy in the real-world, not can they give you V8 driveability.
Ignoring the reliability I'd say the MPi was a compramise, using what was in the parts bin (not a bespoke engine like the TDi).
That said you should buy a Disco on it's condition too. If the MPi is solid, cheap and easy to insure then maybe it's the one for you. I understand they run ok on LPG too, and 20MPG at 50p/litre is way better than the 11mpg my V8 does, or the 28 our TDi manages.