As a heavy lender and borrower on BTCJam let me say the following:
#1 - why would you think its improper to have more then 1 loan at same time? I have 3 cars - and 3 car loans. I qualified for all 3 and by having 3 it lowers my credit score. BTCJam works under the same principle. You can actively see how many loans a person has out and for how much at the time you decide to invest. So this point is null and void.
#2 - there are 2 types of loans - btc loans and fiat linked loans. The poster saying they got back less then they invested because of the change in value of btc - was investing in a fiat linked loan. SImple - dont even invest in fiat linked loans and that is not a problem. So this point is resolved.
#3 - scammers - yes there are scammers. Suggestion is to invest in A and B rated borrowers with a history of repayment. Anyone else invest very cautiously or very small amounts. As for 1 bad loan knocking out 10 good ones? I dont get your math. My average loan investment amount as a lender is around .025 btc per loan at average interest rate of 5% per payment period. If 1 guy doesnt pay me I get it back rather quickly. Out of over 50 loans I have invested in - I have had 3 people default. I am very much in the green.
#4 - Interest rates - they are per payment period. So if you decide to pay daily - you will be paying the choosen interest daily. So if you do a 1 btc loan, paid back daily at 1% - you will be paying 1% daily. If you do 1 btc with weekly payback at 1% - you will be paying back 1% weekly. This is not very clear - they really should make this much more obvious to borrowers and lenders.
Look like every platform BTCJam has its issues - but what makes BTCJam strive is the community. There is a very strong community and it has offshoots that include real life friends, investment clubs, mining groups, google plus hangouts....etc. The members of this community stick together to inform each other about scammers, suspicous activity, improvements to the system etc.
So if your interested in using BTCJam I suggest you stop by the USER community on facebook - which is 100% USER run and ADMINed by USERS.
https://www.facebook.com/groups/btcinvestors/Its a great platform, and yes you can earn significant coin if you are cautious and listen to the community members advice on what to watch out for. Yes there are scammers every once in awhile - and the users and the company are striving to fix that but you will always have jerks in the world.