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Join Polyline tool - Its OK on Arcs and lines, but use sparingly IMO on splines , I dislike all the lines it produces,

using join Polyline on a spline or bexier, converts it into lots of little Polylines, fig  2.1. i.e. toooo many nodes, which can give Genie a headache as she tries to work out what goes where., if you have to use join Polyline then try the convert to curves tool on a copy of the objects to see if its ok, often it will work ok, but sometimes yuk,

Tweaking

Tweaking of the profiles is nearly always needed after the first test loft, , As can be seen from the gray render Fig 2.2, I messed it up,  this has a big bulge at the end. Mine’s around my waist but I’ll not go into that. - simply re-scaling the profiles sorted it out.

A note about  start and finish profiles, the large ‘O’ Fig 2.3 is one loft, and normally one can click  the same single profile to start and finish a closed loft,  but occasionally TC will throw this up as an error, reason - no idea, at least I’m honest, but if this happens simply find a suitable hidden point and create a separation with two profile very close together, do the loft and do a second loft between the two profiles and add together,  

Full profile, closed Spline Fig 2.4

Each profile is N’ SEKE’d to the centre line (blue)  Fig 2.5 and scaled down equally in both X and Y scale boxes, the red spline is just there as a guide to give an indication of how much to scale up / down

A few drawbacks to this method, are - - -

It can takes longer to loft a full profile if it has any complexity so the delays between loft / tweak / loft / tweak can become tedious and lead to errors,

The more complex a  profile  is, - the more nodes it contains, the more chance of errors creeping in,  

 If one needs to alter some nodes its  more difficult to alter both sides exactly the same

Problem - The main problem using a full profile is complexity, by that I mean  more corners & nodes,  

Cause - simply how it was drawn,         yep no one else to blame for this one

Solution - these can often give very good results but if using a full profile causes some problems like going round tight corners, then simplify the profile or choose a different method of lofting is the best solution,  sometimes one can get round some problem by lofting in stages and boolean adding, this may produce seams, but choose a hidden positions or a straight section, sometimes choice of material will dictate if seams are obvious,  Of course this is no good for production purposes, but for that you would probably use a dedicated solid modeling program anyway,  its all in the mind, if people can’t see it - it ain’t there,.