I have a 5 figure SB account with LCG and have been with them years, going back to when they traded as Capital Spreads. They're old, big, UK-based, FCA-regulated - all good indications as far as SB goes.
I don't day-trade with them, but SB is useless for very short time-frame trades so no other SB firm would be any better.
Price spikes occur from what I've seen 1-2 times a year but that's only in the stuff I'm trading - mostly forex and indices. But these are obvious price data input glitches, the firm puts them right immediately, I don't think I've ever had to ask them to do it. So they may have more errors than I've seen but I assume they put them right too else we would have heard about it from every corner.
SB firms are often accused of hunting clients' stops. I don't see LCG doing this. Plus its contrary to their FCA terms so effectively illegal for them. It would be impossible for them to keep it secret. But if the market goes stop-hunting, LCG and everyone else can do nothing but follow price, and if price is volatile they may even have to widen their spreads which is another good reason not to go day-trading by SB with very tight stops.
I know nothing about ECN.